Implement PID highlighting. This can be activated by pressing
'L' and entering a PID to highlight. Highlighting can be cleared
by entering '+' as a PID to highlight. This feature is inspired by
the OpenBSD top, which uses 'P' key for that. We have to use 'L'
because 'P' is already used for toggling per-CPU statistics.
Obtained from: OpenBSD (partially)
MFC after: 2 weeks