Support host count in machinefile

This is the initial support for an improved machinefile, as described in #7589.

- Support the syntax `[user@]host[:port][*count] [bind_addr]` in the
  machinefile.
- Initial code just re-writes the list fed to addprocs() by repeating the host
  as specified, so there is no user-visible change.

Includes basic documentation.