MEDIUM: connection: introduce "struct conn_src" for servers and proxies

MEDIUM: connection: introduce "struct conn_src" for servers and proxies

Both servers and proxies share a common set of parameters for outgoing
connections, and since they're not stored in a similar structure, a lot
of code is duplicated in the connection setup, which is one sensible
area.

Let's first define a common struct for these settings and make use of it.
Next patches will de-duplicate code.

This change also fixes a build breakage that happens when USE_LINUX_TPROXY
is not set but USE_CTTPROXY is set, which seem to be very unlikely
considering that the issue was introduced almost 2 years ago an never
reported.