Provide userland notification of gpio pin changes ("userland gpio interrupts").

This is an import of the Google Summer of Code 2018 project completed by
Christian Kramer (and, sadly, ignored by us for two years now). The goals
stated for that project were:

FreeBSD already has support for interrupts implemented in the GPIO
controller drivers of several SoCs, but there are no interfaces to take
advantage of them out of user space yet. The goal of this work is to
implement such an interface by providing descriptors which integrate
with the common I/O system calls and multiplexing mechanisms.

The initial imported code supports the following functionality:

• A kernel driver that provides an interface to the user space; the existing gpioc(4) driver was enhanced with this functionality.
• Implement support for the most common I/O system calls / multiplexing mechanisms:
  • read() Places the pin number on which the interrupt occurred in the buffer. Blocking and non-blocking behaviour supported.
  • poll()/select()
  • kqueue()
  • signal driven I/O. Posting SIGIO when the O_ASYNC was set.
• Many-to-many relationship between pins and file descriptors.
  • A file descriptor can monitor several GPIO pins.
  • A GPIO pin can be monitored by multiple file descriptors.
• Integration with gpioctl and libgpio.

I added some fixes (mostly to locking) and feature enhancements on top of
the original gsoc code. The feature ehancements allow the user to choose
between detailed and summary event reporting. Detailed reporting provides
a record describing each pin change event. Summary reporting provides the
time of the first and last change of each pin, and a count of how many times
it changed state since the last read(2) call. Another enhancement allows
the recording of multiple state change events on multiple pins between each
call to read(2) (the original code would track only a single event at a time).

The phabricator review for these changes timed out without approval, but I
cite it below anyway, because the review contains a series of diffs that
show how I evolved the code from its original state in Christian's github
repo for the gsoc project to what is being commited here. (In effect,
the phab review extends the VC history back to the original code.)

Submitted by: Christian Kramer
Obtained from: https://github.com/ckraemer/freebsd/tree/gsoc2018
Differential Revision: https://reviews.freebsd.org/D27398