date: Add vary.[ch] from FreeBSD 9

Imported vary.c and vary.h from FreeBSD 9.2 in preparation for porting
support for vary (command line option -v).

This commit serves as base-line. Neither file has been modified from
its original form (meaning vary.c won't compile on non-FreeBSD systems)
and neither file is being included in the build yet.

Modifications to include it in the build will be made as follow-up
commits.

vary.c and vary.h can be found on the web under these URLs:

http://svnweb.freebsd.org/base/release/9.2.0/bin/date/vary.c?view=log
http://svnweb.freebsd.org/base/release/9.2.0/bin/date/vary.h?view=log
date: Fix vary.c so it'll compile on non FreeBSD platforms

The following changes were made:

1. #define __FBSDID if not already defined
2. add __attribute__ ((pure)) to function trans()
3. fix whitespace

The first problem lead to a compiler error due to the undefined symbol.
The second problem lead to a compiler warning, with GCC suggesting
attribute "pure" be added to the function. Since warnings are treated
as errors in coreutils, this, too, prevented vary.c from building.
Lastly, the final change was due to GIT complaining about whitspace
problems.
date: Add vary.[ch] to the build

We add vary.h to noinst_HEADERS and define src_date_SOURCES, since
"date" now depends on more than one source file.
date: Add vary functionality.

We modify date to recognize the command line options -v and --varity
and to call the appropriate vary functions.