Add strftime formatting for ps.boot_time

This also fixes a bug in ps.boot_time which affects newer psutil
releases, where psutil.get_boot_time() has been renamed to
psutil.boot_time().

It also clears up some deprecation warnings on newer psutil versions by
preferring newly-added methods for ps.num_cpus and
ps.total_physical_memory.

Fixes #12428.