Modified username_extension masquerading to only use the extended address on

Modified username_extension masquerading to only use the extended address on
generated [Resent-]From: lines and SMTP envelope From:.  With Neil's original
implementation, nmh's global idea of the username was changed, which would
result in inc lying and saying you had no new mail because it was looking for a
mailbox called, for instance, "dan-nmh" (where username was "dan" and
$USERNAME_EXTENSION was "-nmh").

While in there (adrsprintf()), added checking of snprintf()'s return code and
added calls to adios() when things are not kosher.  Also simplfied the really
confusing REALLYDUMB #ifdef'ing, which didn't even jibe with the comment
explaining REALLYDUMB in acconfig.h.  Now adrsprintf() will always return just
the username, even if an explicit domain is passed in (which currently doesn't
happen anywhere in nmh).