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

When I tried the `check_running_kernel` check on a fresh install of `Debian GNU/kFreeBSD testing (jessie)`, I got the following result:

    root@kfreebsd-01:~# /usr/lib/nagios/plugins/check_running_kernel 
    WARNING: Currently running kernel (GNU/kFreeBSD 10.0-1-amd64 #0 Tue, 20 May 2014 12:47:21 +0100) does not match on disk image (~ 2014-05-22 19:41:24)
    root@kfreebsd-01:~# 

After a short investigation, I noticed that the `uname -v` output contains a time zone, which includes a `+` character - this output is later used in a regular expression, so the `+` from the time zone will be interpreted there, and must therefore be escaped. With this change, I get the expected result:

    root@kfreebsd-01:~# ./check_running_kernel 
    OK: Running kernel matches on disk image: [GNU/kFreeBSD 10.0-1-amd64 #0 Tue, 20 May 2014 12:47:21 +0100]
    root@kfreebsd-01:~#