(CFACT-121) Implement dynamic loading of the ruby dll on Windows

Adds defines for Windows _access and _waccess calls, to make them similar to Linux.
Adds a scoped_error class for Windows system error messages.
Adds execution::which, implemented identically to Facter's method.
Simplifies POSIX api::find_library with use of tie for comparison and assignment.
Implements Windows api::find_library, dynamic_library::load, and dynamic_library::find_symbol to facilitate loading the Ruby dll.
+1 on logging ec errors to debug.

Re the voodoo around X_OK, I'd rather not carry forward (and maintain) ill-understood behavior from past implementations unless it buys us something (and even then only with explanatory comments). But empirically, do we get any desired functionality from this logic?