Ensure that X selection convertors properly encode returned strings.

Ensure that X selection convertors properly encode returned strings.

Though not itself a regression, this bug was exposed by the support
for MULTIPLE selections, which is new to Emacs 24 (see Bug#11315).

* lisp/select.el (xselect--encode-string): New function, split from
xselect-convert-to-string.
(xselect-convert-to-string): Use it.
(xselect-convert-to-filename, xselect-convert-to-os)
(xselect-convert-to-host, xselect-convert-to-user): Ensure that
returned strings are properly encoded.