(latex-block-args-alist, latex-block-body-alist): New vars.

(latex-block-args-alist, latex-block-body-alist): New vars.
(latex-insert-block): Use them.
(tex-string-prefix-p): New fun.
(tex-guess-main-file): Use it to detect when the main file
is in a parent directory.
(tex-main-file): Try to find a main-file in parent directories.
(tex-compile-default): Don't use `gv' on pdf files just because
`gv' was used recently on a ps file.  Remove unused arg `dir'.
Reuse a previous command as-is if it applied to the same file.
(tex-compile): Use the right file name when file is not in dir.