MFC r313958, r319717, r321810

r313958:
Import mandoc 1.4.1rc2

r319717:
Import mandoc snapshot 2017-06-08

It implements missing man(7) macros used in base by kerberos/ntp and makes them
supported by mandoc.

This import should have been done before the removal of groff.

Reported by: gordon

r321810:
Update mandoc to 1.14.2