Add options to make `who' more POSIX compliant.

Add options to make `who' more POSIX compliant.

Accept new options: --all (-a), --boot (-b), --dead (-d),
--login, --process (-p), --runlevel (-r), --short (-s), --time (-t),
--users (-u).
The -u option now produces POSIX-specified results and is the same as
the long option `--users'.  --idle is no longer the same as -u.
(time_string, print_line, print_boottime, print_deadprocs, print_login,
print_initspawn, print_clockchange, print_runlevel): New functions.
(print_user): Renamed from print_entry and reworked.
(search_entries): Remove function.
(who_am_i): Likewise.
(usage): Describe new options.
(main): Handle new options.
Mostly from Michael Stone.