Add timezone and daylight symbols for XSI compatibility.

What's worse, the tzfile.h that gets installed is over 20 years old
and doesn't match the real tzfile.h in libc/time.  This makes the
tree safe for /usr/include/tzfile.h removal.  The TM_YEAR_BASE
define has been moved to time.h temporarily until its usage is
replaced by 1900 in the tree.  Actual removal of tzfile.h is pending
a ports build.  Based on a diff from deraadt@

including CLOCK_{PROCESS,THREAD}_CPUTIME_ID constants and
{clock,pthread}_getcpuclockid() functions.
Worked out at t2k13 with help from tedu@ and matthew@ and testing by aja@
ok matthew@

ok deraadt@

making files stand-alone, some __restrict additions.
tweaks kettenis@; ports testing and fixes landry@ and jasper@
ok millert@

* strptime(3) was introduced in XPG 4.0
* clock_gettime(3) and friends were introduced in 1003.1b-1993
* asctime_r(3) and friends were introduced in 1003.1c-1995
and conditionally provide prototypes and associated types accordingly.
This makes our <time.h> standards compliant except for some functions that are
still missing.
ok guenther@

friends in <time.h>.  The kernel needs access to the associated #defines, but
can't get them from <time.h>, so introduce a new header <sys/_time.h> and
include that from <time.h> for userland and <sys/time.h> for the kernel.
ok matthew@, guenther@, millert@

of NULL was incorrect for c++ compilers that aren't
gcc (or pretend to be gcc like clang).
ok miod@

Kernel and bootblocks still use the old 0L value until all the NULL abuses
in the code are fixed.

Add new sys/_types.h header
Include machine/_types.h or sys/_types.h where applicable

in sys/cdefs.h) instead of _FOO_SOURCE.  Also fix several namespace
pollution issues, including the byte order defines.  OK deraadt@

ok deraadt@

You must have an up-to-date gcc for this!
deraadt@ ok

rescinded 22 July 1999.  Proofed by myself and Theo.

OK mickey@ and discussed with deraadt@

Problem noted by Jean-Baptiste Marchand

section.

__null is a magic constant of integral type that converts to a null pointer
as should be, but warns for ambiguity when used to resolved an overload
between an integral type and a pointer type.

All POSIX libc api now there (to P1003.1c/D10)
(more md stuff is needed for other libc/arch/*)
(setlogin is no longer a special syscall)
Add -pthread option to gcc (that makes it use -lc_r and -D_POSIX_THREADS).
Doc some re-entrant routines
Add libc_r to intro(3)
dig() uses some libc srcs and an extra -I was needed there.
Add more md stuff to libc_r.
Update includes for the pthreads api
Update libc_r TODO

entire trees for testing anyway, I might as well do this intrusive touching
of include files now.  Added openBSD tags.