pw: Rename some enums.

Rename M_PRINT and M_UPDATE to M_SHOW and M_MODIFY to match the
names of the commands they represent. No functional change intended.

MFC after: 3 days
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D45096

(cherry picked from commit a9ea647c29cf510a164947251de0d34f53f2bca0)

pw: Don't silently ignore unparsed command line arguments.

MFC after: 3 days
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D45097

(cherry picked from commit c86119328e6b2cfeb4f9319f6b154524d88caaf4)

pw: Test home directory ownership and mode.

MFC after: 3 days
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D45099

(cherry picked from commit e12b7446bddcb91b869abef6048910cc15185a33)
(cherry picked from commit 2f59ee8287548225e7c71db987574fe043ed1be4)

Approved by: re (cperciva)