Now reads from stdin if no hosts.dat or --hostlist

[samrocketman]

Fixes two bugs:

• Junk being passed into --hostlist not being sanitized like other methods. Fixed.
• Blank list being passed into --hostlist not being detected as an error. Fixed.

Adds a feature:

• Reading the host list from stdin if hosts.dat and --hostlist is not provided in the arguments.

This was cherry picked from https://github.com/russss/Herd/pull/13.