ps(1): Consider references to keywords as immutable

Building on the previous commit that moved the 'width' field from VAR
(structure representing the meaning of a keyword) to VARENT (structure
representing an actual column in a display), it is now possible to
declare the field 'var' of VARENT as a constant reference.

At this point, it could be possible to constify the var[] array
containing all the recognized keywords. However, we do not do it as
a subsequent commit will change the way alias keywords are resolved,
causing their fields to be modified to reflect the final values to use
after alias resolution.

In parsefmt(), forget about allocating a new VAR for the selected
keyword to be pointed by the corresponding column (VARENT), and instead
just keep a pointer to that structure in var[].

MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49611

(cherry picked from commit 1dbb1cca9b8b7874f22c06f5340686361c76da47)