Make it possible for operations to refer to GPIO pins by name

• Try to guess what is provided as a pin spec for -t or for get/set operation: number or name. Fails in case of ambiguity.
• Add -p and -N switches to force pin specification interpretation: -p forces spec to be pin number, -N forces it to be name

Submitted by: Emmanuel Vadot <manu@bidouilliste.com>
Differential Revision: https://reviews.freebsd.org/D5201