Implemented new module (msgproc_module) and module instance (msgproc)

Implemented new module (msgproc_module) and module instance (msgproc)
interface.  This allows each processing module to consume/advertise only
one global symbol, as opposed to 4-5 previously.

The HTML, text, and base64 modules have been moved from the old interface
to the new.  Base64 has furthermore become self-contained.

When the quoted-printable code is module-ified, the decoders.h file
can go away.

parser.h and parser.c were removed as they were specific to the old interface,
used to collect the growing number of public symbols for each module.