Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Move dillon's time conversion functions to a new header <timeconv.h>.
Since they were never documented and have never appeared in a FreeBSD
release, no repo-copy of the header is done. This removes namespace
pollution from <time.h>.

Fix the style of the SCM ID's.
I believe have made all of libc .c's as consistent as possible.

I've been meaning to do this for a while. Add an underscore to the
time_to_xxx() and xxx_to_time() functions. e.g. _time_to_xxx()
instead of time_to_xxx(), to make it more obvious that these are
stopgap functions & placemarkers and not meant to create a defacto
standard. They will eventually be replaced when a real standard
comes out of committee.

Add time_to_int(), int_to_time(), time_to_long(), long_to_time().

Add routines to convert time_t to/from fixed-bit fields. These routines
serve two purposes: (1) so we can maintain backwards compatibility with
protocols (rwhod, dump, etc...) that either assume time_t is 32 bits or
assume sizeof(time_t) == sizeof(int), or make other similar assumptions.
(2) To tag such routines (by the presence of these calls) for future
cleanup/extension work.

The 32->64 routine, time32_to_time() (when time_t is 64 bits, that is),
is defined specifically to implement temporal locality to properly set the
msb bits of a 64 bit time_t quantity, using the 50 year rule. The locality
code has not been implemented yet (and doesn't need to be for a while),
but that is the intent. This will allow us to maintain backwards protocol
compatibility past 2038.

These routines are intended to be platform and time_t agnostic.

MFC after: 1 week