DEBUG turns debug logging on for any non-empty value (closes #159)

DEBUG was read with strconv.ParseBool, so a value such as "yes" stopped
the start. It now follows the code style guide: unset or empty is off,
and any other value, "false" and "0" included, turns debug logging on.
No value of DEBUG stops the start. The test that "yes" fails is replaced
by one covering unset, empty, "false", "0" and "yes". README.md no
longer gives false as the default, and says which values turn it on.

Model: opus-5-5

TODO.md

@@ -24,8 +24,9 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 - 2026-10-19: `DEBUG` unset or empty leaves debug logging off, and any other
   value, `false` and `0` included, turns it on, as the code style guide says
   ([#159](https://git.eeqj.de/sneak/netwatch/issues/159)). No value of it stops
-  the start any more; only `true`, `false`, `1`, `0` and their short forms were
+  the start any more; before, only `1`, `t`, `T`, `TRUE`, `true`, `True`, `0`,
-  accepted before. `README.md` no longer gives `false` as its default
+  `f`, `F`, `FALSE`, `false` and `False` were accepted. `README.md` no longer
+  gives `false` as its default
 - 2026-10-19: the three unit tests of a check whose result is discarded (its
   round given up, the user paused, or the first round) also check that the
   host's history is still empty, its status still `pending`, and nothing was