Split out part of mount-util.c into mountpoint-util.c

Split out part of mount-util.c into mountpoint-util.c

The idea is that anything which is related to actually manipulating mounts is
in mount-util.c, but functions for mountpoint introspection are moved to the
new file. Anything which requires libmount must be in mount-util.c.

This was supposed to be a preparation for further changes, with no functional
difference, but it results in a significant change in linkage:

$ ldd build/libnss_*.so.2
(before)
build/libnss_myhostname.so.2:
	linux-vdso.so.1 (0x00007fff77bf5000)
	librt.so.1 => /lib64/librt.so.1 (0x00007f4bbb7b2000)
	libmount.so.1 => /lib64/libmount.so.1 (0x00007f4bbb755000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f4bbb734000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f4bbb56e000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f4bbb8c1000)
	libblkid.so.1 => /lib64/libblkid.so.1 (0x00007f4bbb51b000)
	libuuid.so.1 => /lib64/libuuid.so.1 (0x00007f4bbb512000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f4bbb4e3000)
	libpcre2-8.so.0 => /lib64/libpcre2-8.so.0 (0x00007f4bbb45e000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007f4bbb458000)
build/libnss_mymachines.so.2:
	linux-vdso.so.1 (0x00007ffc19cc0000)
	librt.so.1 => /lib64/librt.so.1 (0x00007fdecb74b000)
	libcap.so.2 => /lib64/libcap.so.2 (0x00007fdecb744000)
	libmount.so.1 => /lib64/libmount.so.1 (0x00007fdecb6e7000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fdecb6c6000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fdecb500000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fdecb8a9000)
	libblkid.so.1 => /lib64/libblkid.so.1 (0x00007fdecb4ad000)
	libuuid.so.1 => /lib64/libuuid.so.1 (0x00007fdecb4a2000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x00007fdecb475000)
	libpcre2-8.so.0 => /lib64/libpcre2-8.so.0 (0x00007fdecb3f0000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007fdecb3ea000)
build/libnss_resolve.so.2:
	linux-vdso.so.1 (0x00007ffe8ef8e000)
	librt.so.1 => /lib64/librt.so.1 (0x00007fcf314bd000)
	libcap.so.2 => /lib64/libcap.so.2 (0x00007fcf314b6000)
	libmount.so.1 => /lib64/libmount.so.1 (0x00007fcf31459000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fcf31438000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fcf31272000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fcf31615000)
	libblkid.so.1 => /lib64/libblkid.so.1 (0x00007fcf3121f000)
	libuuid.so.1 => /lib64/libuuid.so.1 (0x00007fcf31214000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x00007fcf311e7000)
	libpcre2-8.so.0 => /lib64/libpcre2-8.so.0 (0x00007fcf31162000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007fcf3115c000)
build/libnss_systemd.so.2:
	linux-vdso.so.1 (0x00007ffda6d17000)
	librt.so.1 => /lib64/librt.so.1 (0x00007f610b83c000)
	libcap.so.2 => /lib64/libcap.so.2 (0x00007f610b835000)
	libmount.so.1 => /lib64/libmount.so.1 (0x00007f610b7d8000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f610b7b7000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f610b5f1000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f610b995000)
	libblkid.so.1 => /lib64/libblkid.so.1 (0x00007f610b59e000)
	libuuid.so.1 => /lib64/libuuid.so.1 (0x00007f610b593000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f610b566000)
	libpcre2-8.so.0 => /lib64/libpcre2-8.so.0 (0x00007f610b4e1000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007f610b4db000)

(after)
build/libnss_myhostname.so.2:
	linux-vdso.so.1 (0x00007fff0b5e2000)
	librt.so.1 => /lib64/librt.so.1 (0x00007fde0c328000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fde0c307000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fde0c141000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fde0c435000)
build/libnss_mymachines.so.2:
	linux-vdso.so.1 (0x00007ffdc30a7000)
	librt.so.1 => /lib64/librt.so.1 (0x00007f06ecabb000)
	libcap.so.2 => /lib64/libcap.so.2 (0x00007f06ecab4000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f06eca93000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f06ec8cd000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f06ecc15000)
build/libnss_resolve.so.2:
	linux-vdso.so.1 (0x00007ffe95747000)
	librt.so.1 => /lib64/librt.so.1 (0x00007fa56a80f000)
	libcap.so.2 => /lib64/libcap.so.2 (0x00007fa56a808000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fa56a7e7000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fa56a621000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fa56a964000)
build/libnss_systemd.so.2:
	linux-vdso.so.1 (0x00007ffe67b51000)
	librt.so.1 => /lib64/librt.so.1 (0x00007ffb32113000)
	libcap.so.2 => /lib64/libcap.so.2 (0x00007ffb3210c000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007ffb320eb000)
	libc.so.6 => /lib64/libc.so.6 (0x00007ffb31f25000)
	/lib64/ld-linux-x86-64.so.2 (0x00007ffb3226a000)

I don't quite understand what is going on here, but let's not be too picky.