Fix various syntax errors in the built-in equations contributed by
@Wiljea.
Also some changes in units:
* use `Ω` (Unicode 937) character instead of `Ω` (Unicode 8486) for
the Ohm unit. There is a dedicated Unicode codepoint, but since we
don't use special characters for `m` or `s` units, there is no
reason we should make things more difficult for Ohm.
* Use metric units consistently, i.e. replace `_lb` with `_kg`, `_in`
with `_cm`, `_lbf` with `_N` and so on. It might be worth it later
to make a metric and an imperial variant of this file.
* Use degrees for user angles input in equations.
* Rename variables that are reserved words, e.g. `Re` and `vx`.
This is because DB48X is by default case-insensitive.
Note that we could have a prefix character forcing a symbol for
reserved names (much like we have a prefix for constants).
Fixes: #1162
Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
4c75e9c894