#356 setgroups() not called before setuid()

[Dominik Mierzejewski]

When dropping privileges at tree/minidlna.c:1072, setuid() is called without the recommended setgroups() call preceding it.

See the following links for more detailed discussion on why that's recommended.
https://security.stackexchange.com/questions/122141/always-setgroups-before-setuid
CERT C rule POS36-C
Dropping Privileges in setuid Programs

I propose the attached patch to address this issue.