mhparse.c: Use constant indent for second line of messages.

mhparse.c: Use constant indent for second line of messages.

It was using knowledge of inform()'s workings to indent by the varying
width of global invo_name plus two for the following ": ".  This also
cluttered every call's parameters with a strlen(), cast to an int for
printf's width specifier, and an empty string.  All noise for little
gain.