Use explicit-shell-file-name and $ESHELL as default shells

[bcc32]

M-x term and M-x ansi-term both check these two values before using shell-file-name. The description of the variable suggests that explicit-shell-file-name should be used for user-requested shells, whereas shell-file-name may be used (if different) for automatically spawned shells for things like compilation.

Since shell-pop is more like a wrapper around M-x term, it seems appropriate to check these variables first, the same way the interactive form of M-x term does.

[syohex]

Thanks