make: all command line overrides go in .MAKEOVERRIDES

Not all variables that start with '.' are internals,
and unless they are explicitly flagged as internal should go
into .MAKEOVERRIDES

Update varname-dot-makeoverrides to check this.

Also avoid using SCOPE_CMDLINE when ReadOnly will do.

tests/make: reproduce and explain '$' in MAKEFLAGS

https://mail-index.netbsd.org/current-users/2023/02/24/msg043633.html

tests/make: test how '$' behaves in .MAKEOVERRIDES and MAKEFLAGS

make(1): add dummies for fine-grained tests, one per single feature

The test names have been derived from the current manual page.

All these tests are dummies right now, and the code from the existing
tests will be moved into the new tests step by step.

This is done to prevent modmisc, escape, varmod-edge and varmisc from
growing without any bounds, and to reduce the side-effects of one test
to the others.