git-http-backend-verify: provide REQUIRE_SSL_CLIENT_VERIFY_SUCCESS knob

git-http-backend-verify: provide REQUIRE_SSL_CLIENT_VERIFY_SUCCESS knob

If REQUIRE_SSL_CLIENT_VERIFY_SUCCESS has been exported (to any value)
when running the git-http-backend-verify script, then if the request
is a push, `SSL_CLIENT_VERIFY=SUCCESS` will always be required in the
environment.

The sample apache.conf file already checks for this and does not run
the script unless SSL_CLIENT_VERIFY is SUCCESS.  As a result it does
not export SSL_CLIENT_VERIFY since that's a minor efficiency penalty
and it's already checked it.

This script does verify that SSL_CLIENT_VERIFY is SUCCESS for push
operations, but only if SSL_CLIENT_VERIFY is actually set.

Nevertheless, for the security paranoid, setting the environment
variable REQUIRE_SSL_CLIENT_VERIFY_SUCCESS (to any value) before
running git-http-backend-verify will always require SSL_CLIENT_VERIFY
to be set to SUCCESS before allowing a push.

This will necessitate adding "+StdEnvVars" to the "SSLOptions"
directive(s) in order to make Apache export SSL_CLIENT_VERIFY when
running the git-http-backend-verify script.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>