hold messages for unknown users

[Eugene Crosser <crosser@online.ru>]

The problem was not *exactly* what I expected.
My custom getpwnam() returns NULL and errno==0 if there is no such user.
I think that errno==0 may very well be indication of "user does not exist"
condition in "real" unices as well.

My observation on various versions of Solaris show this: depending on
whether MT-safe or non-MT-safe versions of getpwnam[_r]() are used, you
can get for a non-existing user EINVAL or EBADF (sic!).  "Common sense"
tells me that ENOENT and zero might be used to indicate "no error, user
does not exist" condition on other systems.

Of course, all of this is pure black magic...

Eugene