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).