restart_source configuration option is undocumented

The configuration option `global.restart_source` is undocumented in
stb-tester (it's only mentioned in the release notes and a comment in
the system-wide config file) even though this is an important feature
for Hauppauge HD PVR users.

To make users aware of this option, make `restart_source` available as
`--restart-source` command line argument, just like other configuration
options, and mention it in the stbt man page.