Rework the 'make -n -n' feature such that '-n' recurses and '-N' does not.

Bmake has a documented feature of '-N' to skip executing commands which is
specifically intended for debugging top-level builds and not recursing into
sub-directories. This matches the older 'make -n' behavior we added which made
'-n -n' the recursing target and '-n' a non-recursing target.

Removing the '-n -n' feature allows the build to work as documented in
the bmake manpage with '-n' and '-N'. The older '-n -n' feature was also
not documented anywhere that I could see.

Note that the ${_+_} var is still needed as currently bmake incorrectly
executes '+' commands when '-N' is specified.

The '-n' and '-n -n' features were broken for several reasons prior to this.
r251748 made '_+_' never expand with '-n -n' which resulted in many
sub-directories not being visited until fixed 2 years later in r288391, and
many targets were given .MAKE over the past few years which resulted in
non-sub-make commands, such as rm and ln and mtree, to be executed.

This should also allow removing some indirection hacks in bsd.subdir.mk and
other cases of .USE that have a .MAKE by using '+'.

Sponsored by: EMC / Isilon Storage Division
Discussed on: arch@ (mostly silence)