MINOR: proto_reverse_connect: support source address setting

MINOR: proto_reverse_connect: support source address setting

Support backend configuration for explicit source address on
pre-connect. These settings can be specified via "source" backend
keyword or directly on the server line.

Previously, all source parameters triggered a BUG_ON() when binding a
reverse connect listener. This was done because some settings are
incompatible with reverse connect context : this is the case for all
source settings which do not specify a fixed address but rather rely on
a frontend connection. Indeed, in case of preconnect, connection is
initiated on its own without the existence of a previous frontend
connection.

This patch allows to use a source parameter with a fixed address. All
other settings (usesrc client/clientip/hdr_ip) are rejected on listener
binding. On connection init, alloc_bind_address() is used to set the
optional source address.