Locking fixes. The original code had several places where an error-handling path would exit a function with a mutex unlocked that would be re-locked on a normal exit. There was also a place where an SLIST was traversed without holding the mutex that protects additions and deletions on that list.

Primarily style fixes, mostly reformating block comments to the freebsd standard.

Also, several device_printf()s are now controlled by #ifdef GPIOC_DEBUG, so that there are normally no printfs done by the interrupt handler except when debugging is specifically enabled.

Allow only edge-triggered interrupts, and only if they are supported on that pin according to the capabilities flags from the hardware driver.

This update adds the option to choose between detailed (the default) and summary reporting, and removes the restriction that only one pin-change event can occur between each call to read(2) without losing data. It also adds a new test/demo program, based on the test code written by Christian for the gsoc project.

Detailed reporting reports each pin change event by reporting the pin number, timestamp of the event occurrance, and the pin state at the time of the event (useful when watching for both edges); a circular buffer is used to handle multiple events between read(2) calls. Summary reporting reports the pin number, the timestamp of the first event on that pin, the timestamp of the most recent event on that pin, and the number of pin-change events that occurred since the last read(2) call.

While working on the detailed vs summary reporting logic, I realized that the original gsoc code only allowed a single pin-change event to be recorded between each call to read(2), no matter how many pins were being monitored. If two pins changed simulataneously, you could only be notified about one of them; the other would be lost. These changes add a fifo (actually a circular buffer) to store multiple events between read() calls. By default the fifo size is 2*numpins where numpins is the number of pins supported by the hardware. In effect, this most usually results in a 64-event fifo on most modern hardware that organizes gpio pins into banks of 32 pins. Even on one-big-bank hardware such as Allwinner SoCs that have 150 pins, that's only 300 events (times 16 bytes per event, so not a lot of memory wasted).

A new ioctl is available to choose between detailed or summary reporting, and to optionally increase the size of the event fifo.

A few last minor tweaks to the test program.