mail/: Add new (temporary) libaries for which to test Rmail/mbox such

mail/: Add new (temporary) libaries for which to test Rmail/mbox such
that Rmail/babyl is not affected.  This creates a facility/feature
called "pmail" (analagous to "rmail") that can be used independently
from Rmail for testing purposes.  The plan is to replace the "rmail"
files eventually and remove "pmail" entirely at that point.  In the
interim, interested developers can use either Rmail or Pmail or both
(which is not recommended for the casual User or the faint of heart).