Make unixify unescape filespecs already in Unix format.

Make unixify unescape filespecs already in Unix format.

Sometimes filename components get pasted together in ways that put
a component with caret escapes proper to Extended Filename Syntax
(EFS) for native filespecs into a Unix-format filespec. An example
would be /a/b/foo^_bar, where the '^_' sequence indicates a space
in a native filespec but is technically invalid in a Unix
filespec.

But it doesn't really cost much more to remove the escapes since
we're copying the whole string anyway, so go ahead and do that.
This could theoretically cause trouble if we get Unix-format paths
that have literal carets in them, but that theoretical trouble is
currently less troubling than the practical trouble that crops up
various places in the test suite if we don't do this.