top: add -p option and p command to only show a single process

Allow to show only a single process specified by PID. This could
be done either by running top like 'top -p PID' or using the 'p' command
inside top.

Reviewed by: eadler
Approved by: eadler
Obtained from: OpenBSD
Differential Revision: https://reviews.freebsd.org/D15501