BACKPORT: util/xmlconfig: add MESA_DRICONF_EXECUTABLE_OVERRIDE

Allow the loading process to affect driconf option matching without
changing the behavior throughout mesa common code or leaking the name of
the loading process to logs, artifact storage, or in sub-thread naming,
as can be the case with the broader MESA_PROCESS_NAME override.

This new MESA_DRICONF_EXECUTABLE_OVERRIDE takes higher precedence over
MESA_PROCESS_NAME in the case where both are set.

Backport notes:
* changed formatting of docs/envvars.rst to conform to old conventions
* added `#include "os_misc.h"`

BUG=b:266008251
TEST=vkcube in VM; check host-visible thread names

Change-Id: Ifc64a935163d816f7c82e5f6b9a50b63f2c5ee41
Signed-off-by: Ryan Neph <ryanneph@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20779>
(cherry picked from commit 65adf0c0af5b91f7ea3a9754556540ede40d9c14)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/mesa/+/4220891
Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org>
3 files changed