Allow a comma-separated list in login class capabilities,
by adding a version of strcspn that allows quoting.

PR: 236204
Differential Revision: https://reviews.freebsd.org/D25368

(cherry picked from commit f32db406504ece1b28f43dc816736e081fe22826)
(cherry picked from commit f122e552354e84f18b82d11e40c6f8f214ed8abc)