Refactor the "check categories and severity" logic in diag.t

Move the "todo" bailout as early as possible, before any work is done.
Replace C<use 5.01> [sic] with C<use feature 'state'>, as this makes the
intent much clearer.
Use parenthesis for like() and is() as this is consistent with the rest of the
test, and permits a simplification of the BEGIN handling.
Avoid VMS running autodoc.pl twice for no reason.

Previous descrip_mms.template carried separate rules to build perlintern.pod
and perlapi.pod, each of which caused autodoc.pl to run. autdoc.pl builds
both files when it runs, so this was duplicate work.
In t/porting/diag.t, use TestInit to chdir and set up @INC.

Also eliminate the $|=1, as it's really not necessary for tests that neither
fork nor shell out to other processes.
t/porting/diag.t now uses regen/embed_lib.pl to parse embed.fnc

Previously it had its own custom parsing code for embed.fnc.
In regen/regen_lib.pl, add 'Pod' as a third supported 'language'.

Pod needs a commenting style distinct from C and Perl. (ie the empty string)
Refactor autodoc.pl to use regen_lib.pl to generate file headers & footers.

This reduces duplication of code that generates '-*- buffer-read-only: t -*-',
'ex: set ro:' etc.
autodoc.pl now uses regen/embed_lib.pl to parse embed.fnc

Previously it had its own custom parsing code for embed.fnc.
Refactor autodoc.pl to read MANIFEST line by line, instead of slurping it.