Rework MAKEDEV:

* Use "mknod -F netbsd -r" to create nodes, instead of
"rm ; mknod; chmod; chown".
This means permissions & ownership of existing nodes will
not be changed.
This is up to 30% faster when populating an empty /dev,
and nearly 2x faster when re-running on an existing /dev.

* New options:
-f force change of permission & ownership of existing
devices
-m mknod override name/path of mknod program
(which defaults to $TOOL_MKNOD, then "mknod").
-s generate mtree(8) specfile instead of creating devices

* Remove /usr/etc from $PATH; not needed anymore.

* Provide functions to create devices & directories:
mkdev name [b|c] major minor [mode{=600} [gid{=0} [uid{=0}]]]
create device node `name' with the appropriate permissions
lndev src target
create a symlink from src to target
makedir dir mode
create directory with appropriate mode

* UIDs and GIDs are hardcoded in at MAKEDEV generation time.
(Unfortunately there's not a simple way of determining a GID
a la "id -n user" for determining a UID).

This was tested by generating MAKEDEV for each MACHINE,MACHINE_ARCH
combination and comparing the results of "MAKEDEV all" from the
previous version to the new one.
(This testing actually highlighted mistakes in the previous configuration!)


Simplify distrib/common/Makefile.makedev to use "MAKEDEV -s"