Idea: support multiple input files with -iL

[dmiller-nmap]

Currently, Nmap only supports reading target specs from one file OR from the command-line. We should be able to support multiple files AND command-line specs at the same time. Some potential implementation ideas:

• -iL option processing changes to not open the file immediately, but to put it into a list or vector of filenames, potentially after a stat() check for readability.
• grab_next_host_spec changes to iterate over both the list of command-line expressions as well as the list of filenames from the -iL options, opening each in turn and yielding expressions from those with the existing read_host_from_file function.