SERVER-7496 Handle non-ASCII JavaScript filespecs better in Windows

In Windows, read Unicode environment variables for HOMEDRIVE and HOMEPATH,
use Unicode functions to check for file existence in the shell, use Unicode
functions to parse filespecs when trying to execute JavaScript files, use
Unicode function to create empty '.mongorc.js' on first run of the shell.

Conflicts:
	src/mongo/shell/dbshell.cpp