usr.bin/calendar: do not treat // in text as comment

The C++-style comment marker "//" has been added with the rewrite of
the preprocessor features. Since this character sequence occurs in
ULRS, the reminder of the URL was considered a comment and stripped
from the calendar line.

Change parsing of "//" to only start a comment at the begin of a line
or when preceeded by a white-space character.

PR: 256455
Reported by: Philippe Michel (philippe.michel7 at free.fr)
MFC after: 3 days

Add regression tests for conditions and comments

Fix one case where #else was not corerctly processed and simplify the
conditions logic.

Fix parsing of day and month names in the locale specified in the calendar
file. The previous version would expect those names to match the locale of
the user.

Mention that comments are now correctly processed and that // is supported
in addition to /* ... */.

MFC after: 3 days