custom mount directory support + argument path translation

• support for custom mounting root path (get it from /etc/wsl.conf)
• replace Windows-style paths in command arguments (like C:\Users) to WSL paths (/mnt/c/Users)
• return exit code of executed command

(thanks to hustlahusky for above features)

• fixed a cmd.exe error message when installing
• added messages related to updating wsl-alias in installer script