windows.c: use ANSI for subsurface_unset_conf()

We may not write config variables with UTF-8 characters so the wchar_t
conversation in subsurface_unset_conf() is not needed.

This patch also attempts to improve subsurface_get_conf_bool()
and subsurface_get_conf_int() or better consitentcy with the other OS
files. For both functions return -1 if config key is not found.
Previouosly there was a check for that in  function get_from_registry().

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix saving of po2 values

It was very broken, although it just happened to work for the values
dirk had in his own XML file (0 and 0.800 bar).  Because of confusion
with number of digits it would save 1.080 bar as 1.80 bar.

Use our "show_milli()" helper for showing things that are in milli-units
(which is what we tend to use for most of our values: mK, mbar, mm, ml)

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>