ps: add lots of sorting functions

ps: add lots of sorting functions

Use NULL in format_array to indicate that the print function shall be
used also for sorting. Change sr_nop() to NULL for all fields which
don't use pr_nop() for printing.

Before the commit (note that '--sort label' has no effect: the rows
are not sorted according to label field):
$ ps -A -o command,label --sort label | grep /lib/systemd
/lib/systemd/systemd-journa system_u:system_r:syslogd_t:s0
/lib/systemd/systemd-udevd  system_u:system_r:udev_t:s0-s0:c0.c1023
/lib/systemd/systemd-networ system_u:system_r:systemd_networkd_t:s0
/lib/systemd/systemd-resolv system_u:system_r:systemd_resolved_t:s0
/lib/systemd/systemd-timesy system_u:system_r:ntpd_t:s0
/lib/systemd/systemd-logind system_u:system_r:systemd_logind_t:s0
/lib/systemd/systemd --user user_u:user_r:user_t:s0
/lib/systemd/systemd --user root:sysadm_r:sysadm_t:s0-s0:c0.c1023
grep /lib/systemd           user_u:user_r:user_t:s0

After the commit, '--sort label' works and the output is sorted:
$ ps -A -o command,label --sort label | grep /lib/systemd
/lib/systemd/systemd --user root:sysadm_r:sysadm_t:s0-s0:c0.c1023
/lib/systemd/systemd-timesy system_u:system_r:ntpd_t:s0
/lib/systemd/systemd-journa system_u:system_r:syslogd_t:s0
/lib/systemd/systemd-logind system_u:system_r:systemd_logind_t:s0
/lib/systemd/systemd-networ system_u:system_r:systemd_networkd_t:s0
/lib/systemd/systemd-resolv system_u:system_r:systemd_resolved_t:s0
/lib/systemd/systemd-udevd  system_u:system_r:udev_t:s0-s0:c0.c1023
/lib/systemd/systemd --user user_u:user_r:user_t:s0
grep /lib/systemd           user_u:user_r:user_t:s0

Signed-off-by: Topi Miettinen <toiwoton@gmail.com>