Save client IP address text for "WebIRC" users

Save client IP address text for "WebIRC" users

This patch introduces a new field in the CLIENT structure, "ipa_text",
which points to an optional textual representation of the client IP
address (or NULL) which can be used to store the "real" IP address
information of a client using the "WEBIRC" protocol.

Without this patch, ngIRCd ignored the <ip-address> paramater ...

In addition, the functions Client_SetIPAText() and Client_IPAText()
have been introduced to set and get the textual representation of the
client IP address.

Client_IPAText() can be used even when no "IP address text" has been
set before, it then returns the real IP address of the connection.

Closes bug #159.