MFC r277643:
Remove Giant from /dev/mem and /dev/kmem.

MFC r277743:
Arm: ensure that _tmppt KVA is used exclusively.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add new privileges, PRIV_KMEM_READ and PRIV_KMEM_WRITE, used in opening
/dev/kmem and /dev/mem (in addition to traditional file permission checks).
PRIV_KMEM_READ is different from other PRIV_* checks in that it's allowed
by default.

Reviewed by: kib, mckusick

Add reader/writer lock around mem_range_attr_get() and mem_range_attr_set().
Compile sys/dev/mem/memutil.c for all supported platforms and remove now
unnecessary dev_mem_md_init(). Consistently define mem_range_softc from
mem.c for all platforms. Add missing #include guards for machine/memdev.h
and sys/memrange.h. Clean up some nearby style(9) nits.

MFC after: 1 month

Fix module builds for i386 and amd64.

Add module versions.

Break out the MI part of the /dev/[k]mem and /dev/io drivers into
their own directory and module, leaving the MD parts in the MD
area (the MD parts _are_ part of the modules). /dev/mem and /dev/io
are now loadable modules, thus taking us one step further towards
a kernel created entirely out of modules. Of course, there is nothing
preventing the kernel from having these statically compiled.