Fix some socket leaks in get_srcaddr.
Set source and destination interfaces as rtattrs.

Scans using the -e option could use the correct interface but the wrong
source address.

Thanks to Taburiss S for the report and John Bond for help debugging.
http://seclists.org/nmap-dev/2013/q1/269
http://seclists.org/nmap-dev/2013/q1/327