Demote ftime to a compat symbol; don’t install sys/timeb.h.

Demote ftime to a compat symbol; don’t install sys/timeb.h.

ftime is an obsolete variation on gettimeofday, offering only
millisecond time resolution; it was probably a system call in ooold
versions of BSD Unix.  It is the only function declared by
sys/timeb.h.  It was deprecated in POSIX.1-2001 and removed from
POSIX.1-2008.  In glibc 2.31, we marked its declaration with
__attribute_deprecated__.

Demote it to a compat symbol and stop installing sys/timeb.h.
Remove the minimal test of it (time/tst-ftime.c).

Rename time/tst-ftime_l.c to tst-strftime_l.c; this test program has
never had anything to do with ftime, it tests strftime_l.