Add docs/env_switches.tsv, one row per switch with its class (player, path,
switch, diagnostic, test) and a one-line meaning: 257 switches, 70 of which
no doc mentioned before. dm/tools/env_switches.py writes it from the sources
and keeps the hand-written class and meaning; test_env_switches.py fails when
the code reads a switch the table does not list, when a row names a switch
nothing reads any more, or when a row has no meaning. It also prints the
names the docs quote that nothing reads.
The check compares only the set of names, so mentioning a switch in a doc,
moving a file or reading a switch in a second place does not fail it. A
macro name stays listed when the code also reads it from the environment
(DM_GAME_ENV overrides the profile's macro of that name).
Remove DM_MOVIE: it set g_flag.movie, which nothing read.
Validation: test_env_switches and test_docs pass; a doc mention of an
undocumented switch passes the check and an unlisted getenv fails it; the
Mac host builds for Mario's profile.
Co-authored-by: v0lumehi <34629096+v0lumehi@users.noreply.github.com>
