Variables inherited from env have incorrect origin 'environment' when -e is set
and the makefile does not modify the variable.

$ ls
makefile
$ cat makefile
$(info at parse time hello=$(hello), origin = $(origin hello))
all:; $(info at build time hello=$(hello), origin = $(origin hello))
$ # this is the latest make from master
$ hello=world ~/src/make/m64/make -e
at parse time hello=world, origin = environment
at build time hello=world, origin = environment
make: 'all' is up to date.
$ hello=world make-4.3 -e
at parse time hello=world, origin = environment
at build time hello=world, origin = environment
make-4.3: 'all' is up to date.

This happens because make goes through env variables and defines related make variables
before the command line switches are processed and make has not figured out yet that
-e is specified.
This patch reads command line switches and sets global variable 'env_overrides' before
going through env variables.

This fix is split into 2 patches. The patches are supposed to be applied in
order, part1 before part2. This split keeps logically independent
changes in separate commits. Hopefully, this will make review easier and allows
to refer to or revert individual commits.