Allow dns-brute.hostlist to be a relative path
Clarify nmap.fetchfile only searches Nmap's paths
Fix usage of nmap.fetchfile in several scripts

Discussion thread: http://seclists.org/nmap-dev/2013/q2/121

Existing behavior preserved and preferred, but absolute and local paths
should also work now. Notably, smb-psexec's locate_file function claimed
to check current directory but did not.