capsicum: introduce cap_rights_is_empty Function

Before this commit, we only had the capability to check if a specific
capability was set (using cap_rights_is_set function). However, there
was no efficient method to determine if a cap_rights_t structure doesn't
contain any capability. The cap_rights_is_empty function addresses
this gap.

PR: 275330
Reported by: vini.ipsmaker@gmail.com
Reviewed by: emaste, markj
Differential Revision: https://reviews.freebsd.org/D42780