Make mktime etc. compatible with __time64_t

Make mktime etc. compatible with __time64_t

Keep these functions compatible with Gnulib while adding
__time64_t support.  The basic idea is to move private API
declarations from include/time.h to time/mktime-internal.h, since
the former file cannot easily be shared with Gnulib whereas the
latter can.
Also, do some other minor cleanup while in the neighborhood.
* include/time.h: Include stdbool.h, time/mktime-internal.h.
(__mktime_internal): Move this prototype to time/mktime-internal.h,
since Gnulib needs it.
(__localtime64_r, __gmtime64_r) [__TIMESIZE == 64]:
Move these macros to time/mktime-internal.h, since Gnulib needs them.
(__mktime64, __timegm64) [__TIMESIZE != 64]: New prototypes.
(in_time_t_range): New static function.
* posix/bits/types.h (__time64_t) [__TIMESIZE == 64 && !defined __LIBC]:
Do not define as a macro in this case, so that portable code is
less tempted to use __time64_t.
* time/mktime-internal.h: Rewrite so that it does both glibc
and Gnulib work.  Include time.h if not _LIBC.
(mktime_offset_t) [!_LIBC]: Define for gnulib.
(__time64_t, __gmtime64_r, __localtime64_r, __mktime64, __timegm64)
[!_LIBC || __TIMESIZE == 64]: New macros, mostly moved here
from include/time.h.
(__gmtime_r, __localtime_r, __mktime_internal) [!_LIBC]:
New macros, taken from GNulib.
(__mktime_internal): New prototype, moved here from include/time.h.
* time/mktime.c (mktime_min, mktime_max, convert_time)
(ranged_convert, __mktime_internal, __mktime64):
* time/timegm.c (__timegm64):
Use __time64_t, not time_t.
* time/mktime.c: Stop worrying about whether time_t is floating-point.
(__mktime64) [! (_LIBC && __TIMESIZE != 64)]:
Rename from mktime.
(mktime) [_LIBC && __TIMESIZE != 64]: New function.
* time/timegm.c [!_LIBC]: Include libc-config.h, not config.h,
for libc_hidden_def.
Include errno.h.
(__timegm64) [! (_LIBC && __TIMESIZE != 64)]:
Rename from timegm.
(timegm) [_LIBC && __TIMESIZE != 64]: New function.

First cut at publicizing __time64_t