gpst: Mangle User-Agent haeder in one place

gpst: Mangle User-Agent haeder in one place

If we really need to override it for *all* requests, let's just do that
in gpst_common_headers(). Although maybe it'd be better just to ensure
that vpninfo->useragent is set appropriately in the first place?

It's not clear what we're gaining by preserving ->urlpath either, since
it never gets used as-is; we only *ever* override it with our own
strings. So we might as well just free the old one and set it.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>