Escape '+' in 'uname -v' output in freebsd_check_running_version()

The output from 'uname -v' is later used in a regular expression, so the '+'
from a time zone would be interpreted there, and must therefore be escaped.
Merge pull request #51 from mstock/bugfix/running-kernel-on-kfreebsd

Escape '+' in 'uname -v' output in freebsd_check_running_version()