Modified the mmailid comment to reflect that there are now two types of

Modified the mmailid comment to reflect that there are now two types of
masquerading (wouldn't want sysadmins to not realize that by allowing one,
they're allowing the other).  I also prefer my new wording of "allow" vs. the
old "turn on" because the latter implied that masquerading would take effect
just by setting that variable (not the case unless /etc/passwd had been
previously modified to have the <fakeusername>s).