strtol(3) can optionally set errno to EINVAL if no conversion could be

performed and FreeBSD (at least) implements thin behaviour.  Add an
explicit test to detect this situation in Dispatcher::parse_params().
(This behaviour difference is not relevant to other uses of strtol()).

This corrects the mishandling of (eg) "CSI m" on FreeBSD.

Closes #129 github pull request.
[keithw -- also fixes same issue with mosh-server on OS X]