Changeset 23334e967dd7 created a workaround for a namespace conflict
with Solaris and derivatives.  After some discussion, the team decided
it would be best to move away from using the "M_" prefix for macros.

This patch was automatically generated by running:
  perl -wpi -e 's/\bM_(\w+)\b/MUTT_$1/g' `find . -name '*.[ch]' -print`
with the exception that sys_socket.h was exempted.  (That file will
be backed out subsequent to this commit.)

Thanks to Andras Salamon for supplying the perl script used to make
this change.