sys: Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Add module build stuff for gpioths(4), a driver for DHT11/DHT22 sensors.

Add gpiokeys to the list of GPIO modules built only if FDT is enabled

When building modules, define make variable OPT_FDT if the kernel config
includes the FDT option. Use OPT_FDT to conditionally compile modules
that require FDT support.

In the past we've gotten away with using the arch name as a proxy for FDT
support in makefile conditional logic, but now mips has some platforms with
fdt support and some without and we need a more direct test.

[gpio] add gpiospi to the gpio module.

Add a PPS driver that takes the timing pulse from a gpio pin. Currently
supports only ofw/fdt systems. Some day, hinted attachment for non-fdt
systems should be possible too.

Revert r299739. That did not make it better.
Instead disconnect gpiokeys from the build until it's fixed and buildable;
the SUBDIR list was not ordered properly anyway ;-)

Add loadable module for gpiokeys

Allow building the GPIO bus and associated bits as modules.

This is primarily done to save a few bytes here and there on embedded
systems with limited flash space for kernels - a very limited (sub-1MB)
space may be available for the kernel and may only support gzip encoding.
The rootfs can be LZMA compressed.