lib/, src/, configure.ac: Use utmpx instead of utmp

lib/, src/, configure.ac: Use utmpx instead of utmp

utmpx is specified by POSIX as an XSI extension.  That's more portable
than utmp, which is unavailable for example in musl libc.  The manual
page specifies that in Linux (but it probably means in glibc), utmp and
utmpx (and the functions that use them) are identical, so this commit
shouldn't affect glibc systems.

Assume utmpx is always present.

Also, if utmpx is present, POSIX guarantees that some members exist:

-  ut_user
-  ut_id
-  ut_line
-  ut_pid
-  ut_type
-  ut_tv

So, rely on them unconditionally.

Fixes: 170b76cdd1a9 ("Disable utmpx permanently")
Closes: <https://github.com/shadow-maint/shadow/issues/945>
Reported-by: Firas Khalil Khana <firasuke@gmail.com>
Reported-by: "A. Wilfox" <https://github.com/awilfox>
Tested-by: Firas Khalil Khana <firasuke@gmail.com>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>