Fix fallout from previous uvm.h cleanup.

- pmap(9) needs uvm/uvm_extern.h.

- x86/pmap.h is not usable on its own; it is only usable if included
via uvm/uvm_extern.h (-> uvm/uvm_pmap.h -> machine/pmap.h).

- Make nvmm.h and nvmm_internal.h standalone.

Round of uvm.h cleanup.

The poorly named uvm.h is generally supposed to be for uvm-internal
users only.

- Narrow it to files that actually need it -- mostly files that need
to query whether curlwp is the pagedaemon, which should maybe be
exposed by an external header.

- Use uvm_extern.h where feasible and uvm_*.h for things not exposed
by it. We should split up uvm_extern.h but this will serve for now
to reduce the uvm.h dependencies.

- Use uvm_stat.h and #ifdef UVMHIST uvm.h for files that use
UVMHIST(ubchist), since ubchist is declared in uvm.h but the
reference evaporates if UVMHIST is not defined, so we reduce header
file dependencies.

- Make uvm_device.h and uvm_swap.h independently includable while
here.

ok chs@

Align major numbers in example modules with sys/conf/majors

Patch submitted by: Aditya Vardhan Padala (silv3r)

branches: 1.1.2; 1.1.6; 1.1.10;
Add a new example kernel module: mapper

Basic implementation of mmap.

Submitted by Akul Pillai.