add openconnect__win32_setenv function to compat.c

add openconnect__win32_setenv function to compat.c

Based on:

* POSIX-compatible `setenv` implementation for Windows: https://stackoverflow.com/a/23616164
* Enabling Windows "secure API" getenv_s and _putenv_s functions in MinGW:
  * Using -DMINGW_HAS_SECURE_API: https://stackoverflow.com/a/51977723
  * By manually defining their prototypes: https://stackoverflow.com/a/51977723
  * Apparently, only newer versions of MinGW follow the MINGW_HAS_SECURE_API flag, and
    autodetecting them is quite hard.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>