Avoid hangs with complex shells/prompts

Fixed hangs which occurred when zsh or a complex $PS1 was the default on a local or remote system. tramp-methods (ssh, sudo, etc.) would firstpass through the default shell before running sh. Now, /bin/sh is the shell exclusively.

These changes are confined to augmenting the tramp-methods autoloads.

(Please let me know if there's some other form in which I should submit this patch, and thanks a lot for all you (pl.) do!)