Redirect svnversion stderr to /dev/null if we cannot determine
the tree version, for example if the tree is checked out with an
outdated svn from ports, but the base system svnlite is built.

Approved by:	kib (mentor)
Note NULL encryption method for GELI

PR:		docs/180551
Submitted by:	r4721@tormail.org
Approved by:	gjb (mentor)