ps(1): find_varentry() to take a name instead of a VAR

The only information that find_varentry() needs and uses is
a keyword/var name. The rest of the fields in the passed VAR are
unused.

Changing its signature will ease introducing new calls to
find_varentry() in subsequent commits, as there no VAR object will exist
to be passed but just a name.

Reviewed by: kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49609

(cherry picked from commit c1e5a7fdad631458768fc45a82b4d43bade8d0c8)