Use 24h timestamps in the ps(1) STARTED column

The previous 12h AM/PM format was perplexing as it didn't follow the
locale of the user and was a minor annoyance to FreeBSD users coming
from Linux. Additionally, the man page was incorrect about the strftime
format.

There are three time formats that may be displayed in the STARTED
column depending on the age of the process. Below is an example.

For a process started at 14:30 on Monday 16 March 2015, the following
formats may be used:

14:30 for process < 24h old (24h Timestamp)
Mon14 for process > 24h, < 1 week old (Weekday Hour)
16Mar15 for process > 1 week old (Day Month Year)

Differential Revision: https://reviews.freebsd.org/D1620
Reviewed by: brd
Approved by: trasz