Fix Debian bug #482947: No --nas-ip-address option

Fix Debian bug #482947: No --nas-ip-address option

| check_radius doesn't seem to provide any way to modify the
| NAS-IP-Address attribute that it uses in the packets it sends, but it
| does so for NAS-Identifier.
|
| Instead, it hardcodes the IP address that it gets from the
| rc_own_ipaddress() library call, and that in turn translates into
| calling gethostbyname() on the result of uname(). This call can easily
| fail, and its result can easily be unsuitable - for example when the
| Nagios instance uses its own virtual host, and you don't want the
| original system hostname leaked to the RADIUS servers you monitor with
| this.
|
| Furthermore, this behaviour is inconsistent with RFC 2865, which
| defines the two attributes as analogous and never suggests hardcoding
| the value of either of them in client software.

Therefore, this commit adds the "-N, --nas-ip-address" option which
allows for specifying the value of the NAS-IP-Address attribute.

| I've also noticed that the original code for NAS-IP-Address hardcoding
| is broken in its error handling - it does "return (ERROR_PC)", which
| is meaningless in the context of check_radius.c. That actually seems
| to be copy&waste from radiusclient-0.3.2/src/radexample.c. :) I fixed
| that.
|
| While debugging, I also took the opportunity to decouple the
| nas-identifier rc_avpair_add() instance from the initial three,
| because this is just bad practice to lump a fourth optional attribute
| into the same block with the required attributes, the error handling
| for which is throwing the same daft message "Out of Memory?"...

[ http://bugs.debian.org/482947 ]

(Contributed by Josip Rodin, forwarded by Jan Wagner.)