setusercontext(): Apply personal settings only on matching effective UID

setusercontext(): Apply personal settings only on matching effective UID

Commit 35305a8dc114 (r211393) added a check on whether 'uid' was equal
to getuid() before calling setlogincontext().  Doing so still allows
a setuid program to apply resource limits and priorities specified in
a user-controlled configuration file ('~/.login_conf') where
a non-setuid program could not.  Plug the hole by checking instead that
the process' effective UID is the target one (which is likely what was
meant in the initial commit).

PR:                     271750
Reviewed by:            kib, des
MFC after:              2 weeks
Sponsored by:           Kumacom SAS
Differential Revision:  https://reviews.freebsd.org/D40351