Prefer TeX distro GS wrappers.

* preview.el (preview-gs-command): Prefer the TeX distro
ghostscript wrapper scripts `rungs' and `mgs' over calling `gs'
directly which requires it to be in PATH.
Update texinfo files for Git.

* doc/install.texi (Configure): Update for Git.

* doc/preview-readme.texi (Availability): Ditto.

* doc/wininstall.texi: Ditto.
Fix spelling errors in tex-buf.el.

* tex-buf.el (TeX-error-file): Fix spelling error.
(TeX-error): Ditto.
Silence some compilation warnings.

* bib-cite.el: Replace `save-excursion'+`set-buffer' with
`with-current-buffer'.

* tex-bar.el: Ditto.

* tex-buf.el: Ditto.

* tex.el: Replace `save-excursion'+`set-buffer' with
`with-current-buffer'.  Delete trailing whitespaces.