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