Allow mailwrapper to use mailer.conf from localbase (respecting LOCALBASE env var if set)

Phabric:	https://reviews.freebsd.org/D412
Reviewed by:	bdrewery
MFC after:	2 weeks
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270675 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f