ps(1): Have parsefmt() take the list of columns to update

This is in preparation for changing the behavior of the '-O' option.

While here, reformat the definition of 'struct varent', fix formatting
of that of 'struct var' and expand slightly their herald comments.

More reformatting/commenting in 'ps.h'.

No functional change intended.

Reviewed by: kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49607