platform: find getopt in alternate osx locations

Some users may use MacPorts instead of Homebrew.  If brew fails to
return a prefix for getopt, check if the user has 'port' installed and
use the MacPort ${prefix} /opt/local.

Avoids an issue where pass terminates because getopt cannot be resolved
in the assumed location /usr/local/bin.  Provide support to resolve
absolute path of getopt from a MacPorts installation.

Signed-off-by: Jamie Couture <jamie.couture@gmail.com>