various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

r309232: Fix an obvious typo.
r309233: Eliminate variables that are computed, assigned but
         never used.
r309234: pages and psize are always assigned, so there is no
         need to initialize them as zero.

Remove useless check for leading blanks in the month name.  The
code didn't adjust len after stripping blanks so even if a month
*did* start with a blank we'd end up copying garbage at the end.
Also convert a malloc + memcpy to strdup and fix a memory leak in
the wide char version if mbstowcs() fails.
Originally from Andre Smagin.

Obtained from:  OpenBSD (CVS rev. 1.2, 1.3)
MFC after:	1 week

sort: style knits / cleanups.

Our style guide(9) specifies that in absence of local variables
an empty line must be inserted.

Pointed out by:	eadler

Obtained from:	OpenBSD

Obtained from:	OpenBSD

Requested by:	Oleg Moskalenko <mom040267@gmail.com>

- Allow larger sort memory on 64-bit platforms

Submitted by:	Oleg Moskalenko <oleg.moskalenko@citrix.com>

- Do not use mmap() by default; it can be enabled by --mmap
- Add some minor optimizations for -u
- Update manual page according to the changes

Submitted by:	Oleg Moskalenko <oleg.moskalenko@citrix.com>