Add support for additionnal vendor specific headers in messages

Add new API to GSSDPClient to add custom headers to every message
sent. It allows support for applications that need non-standard
headers.

https://bugzilla.gnome.org/show_bug.cgi?id=729613
Replace usage of ?:

Some compilers or settings don't really like that.

Signed-off-by: Jens Georg <mail@jensge.org>