Bug 4437

I recently came across a situation where I wanted to combine the
virtual-config-dir with spamd's setuid abilities.  Basically, this server
handles mail for multiple domains, and that mail is stored and processed under
different user accounts.

Specifically, mail is stored as /home/<username>/<domain>/mail/<mail-user> - for
example, jagerman@jagerman.com mail is located at
/home/jagerman/jagerman.com/mail/jagerman, and is owned by the <jagerman> user.

To get this working with spamd, I used the
--virtual-config-dir=/var/mail/%d/mail/%l/.spamassassin spamd option, then
called spamc with -u user@domain.com - and to make this work, I created a
/var/mail/domain symlink to the appropriate /home/username/domain directory.

The problem, however, is that spamd doesn't handle any setuid handling with this
configuration, which means any .spamassassin/ files it creates (including the
directory itself, if it doesn't exist) will be owned by whatever user spamd is
running as (it was root, in my case while testing).

The following patch addresses that problem by adding a new option
(--virtual-setuid-separator) to spamd that allows spamc to prefix the virtual
username with a system username and fixed string separator.  Additionally, the
patch adds a '%h' escape to the --virtual-config-dir pattern so that the above
symlink directory I described above (/var/mail) is not needed.

In my case, with this patch applied, I start spamd with:

-x --virtual-config-dir=%h/%d/mail/%l/.spamassassin --virtual-setuid-separator=::

I then run spamc with: -u jagerman::test@jagerman.com and spamd setuid()'s to
the 'jagerman' user and uses /home/jagerman/jagerman.com/mail/test/.spamassassin
for its files.