[mmssrc] no support for socks proxies

Submitted by Bastien Nocera `@hadess`

mmssrc (and libmms) don't seem to support socks proxies.

(my pretty bad) reproducer:

Setup a socks proxy on a remote machine (I'm in France on holidays, ssh'ing to a UK-based machine):
ssh -N -f -D 8080 myhomemachine.org.uk
Setup the proxy (socks in gnome-network-preferences, set to "localhost:8080")
Use epiphany to access http://www.ip-adress.com/
Yay, based in the UK (going to the BBC iPlayer page works)
Try to use a Windows Media stream on the BBC website that would be restricted to the UK: no workie, because libmms/mmssrc doesn't have socks5 proxy support.

The socks proxy settings are recorded in GConf under /system/proxy/socks_host and /system/proxy/socks_port

Right now, I'm using tsocks, and LD_PRELOAD tricks:
http://tsocks.sourceforge.net/

(See also bug 505452)