cmd_run: Support per-execution mailboxes

Add support for specifying a list of mailboxes per-execution, using the new -m/--mboxes option. The option accepts a whitespace-separated list of mailbox paths/URLs.

This allows different executions to be triggered off different changed sources, including one having source files specified and another not. In addition to being the default, the latter can also be specified with an empty string. A string of whitespace specifies "no changed files".