calendar(1): Add support for #include with absolute paths

Prior to the addition of cpp support into calendar itself
#include </usr/share/calendar/calendar.all>
was a legal construction in a calendar file.

Permit this again
mtest(8): use correct macro in mdoc

The An macros is used for authors while the Ar macro is used for arguments.
AFAIK mcast-addr and ifname are not authors.

PR:		docs/184649
Submitted by:	cnst++
MFC After:	3 days