Manage auto insertion of labels.

* latex.el (LaTeX-auto-insert-label): New customizable variable.
(LaTeX-label): Use it.

* doc/auctex.texi (Environments): Document
`LaTeX-auto-insert-label'.

* doc/changes.texi: Mention `LaTeX-auto-insert-label'.  Fix a
couple of bad-boxes in the PDF output of the manual.
Keep error overview window if View command is run.

* tex-buf.el (TeX-command): Keep the frame and the buffer
associate to the error overview if the command to be run is View.