um: port_user: Allow setting path to port-helper using UML_PORT_HELPER envvar

This is useful when the uml-utilities user-space package has not been
installed by the administrator and an unprivileged user wants to be able to
telnet into a UML instance. The user can install the port-helper binary to
a writable path and set UML_PORT_HELPER. Fallback to using hardcoded path to
port-helper if environment variable is not set.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Signed-off-by: Richard Weinberger <richard@nod.at>