[gpioths] add kernel modules and correct few issues

[mizhka]

Summary

Hi,

This patch adds kernel module for gpioths driver to avoid change of kernel configuration (thanks to @joel for idea).
Also it fixes following issues:

• typo in comment: no more -> no less (thanks to @rpokala)
• stop callout in function gpioths_detach() (issue found during kldunload testing)
• add critical_enter/exits to avoid preemption (thanks to Maxim Logunov, <spitty.pnz at gmail.com>)

ToDo:

• man page
• find way how to set hints without kernel conf change

Test Plan

Tested on Onion Omega.

[rpokala]

sys/dev/gpio/gpioths.c

216

Was removal of DELAY(1); intentional?

373

How were they removed?

sys/modules/gpio/gpioths/Makefile

32

There seems to be a preference for using ${SRCTOP} rather than ${.CURDIR}. So:

.PATH: ${SRCTOP}/sys/dev/gpio

37

Similarly:

CFLAGS+= -I. -I${SRCTOP}/sys/dev/gpio