Move 'is_regular_file' from common-utils.c to filestuff.c

Move 'is_regular_file' from common-utils.c to filestuff.c

There is no reason for 'is_regular_file' to be in common-utils.c; it
belongs to 'filestuff.c'.  This commit moves the function definition
and its prototype to the appropriate files.

The motivation behind this move is a failure that happens on certain
cross-compilation environments when compiling the IPA library, due to
the way gnulib probes the need for a 'stat' call replacement.  Because
configure checks when cross-compiling are more limited, gnulib decides
that it needs to substitute the 'stat' calls its own 'rpl_stat';
however, the IPA library doesn't link with gnulib, which leads to an
error when compiling 'common-utils.c':

  ...
  /opt/x86-core2--musl--bleeding-edge-2018.09-1/bin/i686-buildroot-linux-musl-g++  -shared -fPIC -Wl,--soname=libinproctrace.so -Wl,--no-undefined -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os      -I. -I. -I./../common -I./../regformats -I./.. -I./../../include -I./../gnulib/import -Ibuild-gnulib-gdbserver/import -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable -Wno-sign-compare -Wno-narrowing -Wno-error=maybe-uninitialized  -DGDBSERVER \
   -Wl,--dynamic-list=./proc-service.list -o libinproctrace.so ax-ipa.o common-utils-ipa.o errors-ipa.o format-ipa.o print-utils-ipa.o regcache-ipa.o remote-utils-ipa.o rsp-low-ipa.o tdesc-ipa.o tracepoint-ipa.o utils-ipa.o vec-ipa.o linux-i386-ipa.o linux-x86-tdesc-ipa.o arch/i386-ipa.o -ldl -pthread
  /opt/x86-core2--musl--bleeding-edge-2018.09-1/lib/gcc/i686-buildroot-linux-musl/8.2.0/../../../../i686-buildroot-linux-musl/bin/ld: common-utils-ipa.o: in function `is_regular_file(char const*, int*)':
  common-utils.c:(.text+0x695): undefined reference to `rpl_stat'
  collect2: error: ld returned 1 exit status
  Makefile:413: recipe for target 'libinproctrace.so' failed
  make[1]: *** [libinproctrace.so] Error 1
  ...

More details can also be found at:

  https://sourceware.org/ml/gdb-patches/2018-09/msg00304.html

The most simple fix for this problem is to move 'is_regular_file' to
'filestuff.c', which is not used by IPA.  This ends up making the
files more logically organized as well, since 'is_regular_file' is a
file operation.

No regressions found.

gdb/ChangeLog:
2018-09-12  Sergio Durigan Junior  <sergiodj@redhat.com>

	* common/common-utils.c: Don't include '<sys/stat.h>'.
	(is_regular_file): Move to...
	* common/filestuff.c (is_regular_file): ... here.
	* common/common-utils.h (is_regular_file): Move to...
	* common/filestuff.h (is_regular_file): ... here.