setclassenvironment: trim leading spaces in variable names

Trim leading spaces in variable names when the list is e.g.
pretty-formatted in /etc/login.conf or ~/.login_conf.

PR: 247947
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D25649

(cherry picked from commit eeaf9d562fe137e0c52b8c346742dccfc8bde015)