w: Use POSIX <utmpx.h> functions where available

<utmp.h> has been deprecated since 2001 in favour of <utmpx.h>.

On glibc systems, utmp is just an alias to utmpx, so there is no
functional change using one over the other.

However, on the musl libc, a library (utmps) can be used to provide
utmpx functionality - but not utmp.  This means that procps either
doesn't work properly (`w` shows nothing under musl with default no-op
implementation), or fails to build (utmps provides utmpx.h but no
utmp.h).

This commit will use utmpx.h where available, which allows `w` to work
correctly with utmps and has no change on glibc systems.
1 job for utmpx-support in 1 minute and 43 seconds
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