[dev] [surf] patch - user agent

Hello,

Here is a patch that adds a _SURF_AGENT property in the same manner as
_SURF_URI and _SURF_FIND, that allows you to change your useragent
while surfing. I find it works best if you have a file with preset
useragents, one per line, which you feed into dmenu, or use some
external script to set it.

Hope this helps,
-emg