Fix some FIXMEs.

* style/paralist.el: Update copyright years.
("paralist"): Use `LaTeX-provided-package-options-member' to
conditionally define environments.
Prevent an infinite loop in TeX-command-expand.

* tex.el (TeX-view-command-raw): Throw an error when `spec' is
nil, otherwise when the function returns nil `TeX-command-expand'
enters an infinite loop.
Ask for TeX-master if unknown.

* tex-buf.el (TeX-command-master): Ask for TeX-master if it cannot
be determined otherwise.
Enable TeX-PDF-mode by default.

* doc/auctex.texi: Mention that `TeX-PDF-mode' is enabled by
default.

* tex.el (TeX-PDF-mode): Enable TeX PDF mode by default.
Add new style/exercise.el.

* style/exercise.el: New file.

Signed-off-by: Tassilo Horn <tsdh@gnu.org>