Refactor: libstonithd: Avoid snprintf() in stonith__action_create()

To avoid the need to error-check snprintf() explicitly, which we have
not been doing. This also avoids imposing an arbitrary (albeit
reasonable) limit of 512 characters on action name length. Pacemaker
technically supports arbitrary action names.

Ref T974

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>