Low: executor: work around flag value mistake

Improve the documentation for enum lrmd_call_options values. They were intended
to be bit flags, but lrmd_opt_drop_recurring was mistakenly set to a
combination of bit values (including lrmd_opt_notify_orig_only).

Thankfully it doesn't affect cluster operation since current internal code sets
only one flag for any given request, and only sets those two values for
different types of requests.

However, external callers could theoretically be affected. As a workaround, use
pcmk_all_flags_set() to check for lrmd_opt_drop_recurring so that it won't be
hit if only lrmd_opt_notify_orig_only is set.

Also, use pcmk_is_set() instead of bitwise comparisons for readability.