gconf: Move getting proxy settings to misc.h/misc.c only

This moves all usage of GConf to misc.c, so it's easier to refactor
later. Also, depending on the OS, the proxy settings might be stored
in another place, so this makes it easier to support different OSes.
Merge pull request #2 from thp/gconf-only-in-misc

gconf: Move getting proxy settings to misc.h/misc.c only