[SV 64803] Set origin for unmodified variables after -e

[SV 64803] Set origin for unmodified variables after -e

Ensure the origin of all variables inherited from the environment is
"environment override" if -e is given.  Previously only variables
that were set in the makefile had this origin.

PDS: Most of these changes are from Dmitry but I slightly modified
the algorithm: instead of rearranging the way in which MAKEFLAGS is
parsed we reset the env_override value to the default before we
re-parse MAKEFLAGS, then we set the origin of all env vars to the
correct value based on the new setting.

* NEWS: Mention the change for backward-compatibility.
* src/main.c (main): Ensure MAKEFLAGS is always marked special.
(reset_makeflags): Set env_overrides back to default before parsing
MAKEFLAGS.
(decode_switches): Call reset_env_override() to check for changes.
* src/variable.h (reset_env_override): Declare a new function.
* src/variable.c (reset_env_override): Go through all env variables
and ensure the origin is correct based on env_overrides.
(set_env_override): Helper function for the hash.
* tests/scripts/functions/foreach: Fix tests.
* tests/scripts/functions/let: Ditto.
* tests/scripts/functions/origin: Ditto.
* tests/scripts/options/dash-e: Add tests.