Don’t include sys/time.h from sys/timex.h.

Don’t include sys/time.h from sys/timex.h.

The interfaces defined in sys/timex.h only need struct timeval, not
any of the other things defined in sys/time.h.

While I was at it I moved stuff around so that sysdeps/…/linux/bits/timex.h
defines everything that uapi linux/timex.h defines (as of version 5.0;
alas, we still cannot use linux/timex.h directly) plus the
MOD_CLKA and MOD_CLKB constants.  In particular, the TIME_* constants
are relevant to users of clock_adjtime as well as adjtimex (I presume—
clock_adjtime is not very well documented) so they should be visible
from time.h as well as sys/timex.h.  Conversely, as far as I can tell,
struct ntptimeval was never a kernel interface on Linux, and MAXTC is
not part of the exposed API and also has the wrong value for current
kernels.   Also I removed a thoroughly obsolete bug workaround from
ntp_gettime.c and ntp_gettimex.c.

	* sysdeps/unix/sysv/linux/sys/timex.h: Don’t include sys/time.h.
	Update commentary.  Don’t define MAXTC.  Move definition of
	NTP_API and the TIME_* constants...
	* sysdeps/unix/sysv/linux/bits/timex.h: ...here.
	Allow inclusion only by sys/timex.h and bits/time.h.
	Update commentary.

        * sysdeps/unix/sysv/linux/ntp_gettime.c
        * sysdeps/unix/sysv/linux/ntp_gettimex.c: Remove obsolete
	check for MOD_OFFSET not being defined.

	* scripts/check-obsolete-constructs.py (HEADER_ALLOWED_INCLUDES):
	Update.