Add __wt_getenv to workaround caching in MSVC CRT's getenv implementation

MSVC CRT's getenv caches the environment variables it returns after the first call. I had to switch to the Windows API to avoid the CRT caching.

Implemented __wt_getenv which now allocates memory, and updated callers.

Test: Mac OS X 10.10 & Windows 8.1 compile, and unit test suite runs
Also, adhoc tests for WIREDTIGER_CONFIG with wt.exe.