emacstbl.pl: Add proper header and footer

[8dcc]

Changes the header and footer of the nasmtok.el file generated by emacstbl.pl. This change is needed for closing skeeto/nasm-mode#14, currently being tested in a require-nasmtok (skeeto/nasm-mode@826f881) branch.

For more information about the library header convention in Elisp, see: https://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html

[8dcc]

I also noticed that some preprocessor tokens such as struc, endstruc, istruc, iend and at are not included in the generated nasmtok.el file.

By looking at the following lines, I can see that some other tokens from the nasm-pp-directives list might be missing as well, such as align and alignb: https://github.com/skeeto/nasm-mode/blob/4e670f6dededab858251670aa5459c950f78d867/nasm-mode.el#L528-L534

Please, review this and add them if possible.