Add a `-m month' flag to provide a more convenient interface for
displaying a calendar for a specific month of the current year than
`cal $(date +"%Y") month'. A few minor code cleanups. Set WARNS=1.
(This code is WARNS=5 clean except for "O' modifier used with %B'
strftime format", which is legal in FreeBSD but GCC doesn't know about.)

MFC after: 1 week