Skip GRANT if it already exists

Previously GRANT statements were run on every run because it was safe and did
not have ill effects. This, however, creates false change notifications which
can be a problem. Added "unless" parameter to the Exec to prevent unnecessary
GRANT commands. Also split most of the mysql commands into reusable variables.
The GRANT detection part has been tested using GNU grep and FreeBSD grep.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>