Allow to specify port with --devtools option (fixes #3597)

Note that using `servo --devtools http://example.org` doesn't work. In
that case either the port must be specified or the option moved to the
end. This is done by getopts and is the same for other such options,
e.g. `--profile`.
auto merge of #3612 : robinst/servo/devtools-port-option, r=jdm

Note that using `servo --devtools http://example.org` doesn't work. In
that case either the port must be specified or the option moved to the
end. But this is the same for other such options, e.g. `--profile`.