[libgpiod] Simplify GPIO monitoring

[Kyle Nieman <Kyle.Nieman@xxxxxxxxxx>]

Hello,

In embedded linux systems, GPIO monitoring for edge-events is common.
For example, OpenBMC has several applications/packages that monitor GPIOs for an
extended period of time.

phosphor-gpio-monitor:
        1. Starts a systemd service on GPIO events
        2. Sets a dbus property based upon initial state and on GPIO events
https://github.com/openbmc/phosphor-gpio-monitor

x86-power-control:
        1. Controls system power based on different button presses
https://github.com/openbmc/x86-power-control

However, there seems to be a lot of duplicate code between these applications
related to monitoring in general that looks like it could be reduced.

GPIO monitoring tends to have the following steps:
1. Configure/set up the GPIO line for monitoring
2. Read the initial value of GPIO line
3. Take action based upon initial value
4. Wait for event by monitoring fd
5. Take action on event, then continue monitoring or cleanup

A function could internally complete the common steps regardless of what GPIO
line is being monitored such as steps 1, 2, and 4. The other steps unique to the
GPIO being monitored could be handled by callbacks. The
gpiod_ctxless_event_monitor functions seemed to do this partially, but were
removed along with the rest of the ctxless functions in commit
9b6e6d268671ef2dd00d9a9abe7ba43d14e7a84b
(ctxless: drop all context-less interfaces).
https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/commit/?id=9b6e6d268671ef2dd00d9a9abe7ba43d14e7a84b

Would there be an issue with adding a higher level abstraction to simplify GPIO
monitoring?

Thanks,
Kyle