pcomplete: Generate completions from --help messages

pcomplete: Generate completions from --help messages

* lisp/pcomplete.el (pcomplete-from-help): New function (and hash
table) to get pcomplete candidates from help messages.
(pcomplete-here-using-help): Helper function to define pcomplete for
simple commands
(pcomplete-completions-at-point): Provide annotation-function and
company-docsig properties.
* lisp/pcmpl-git.el: New file, provides pcomplete for Git.
* lisp/pcmpl-gnu.el: Add pcomplete for awk, gpg and gdb, emacs and
emacsclient.
* lisp/pcmpl-linux.el: Add pcomplete for systemctl and journalctl.
* lisp/pcmpl-rpm.el: Add pcomplete for dnf.
* lisp/pcmpl-unix.el: Add pcomplete for sudo and most commands found
in GNU Coreutils.
* lisp/pcmpl-x.el: Add pcomplete for tex, pdftex, latex, pdflatex,
rigrep and rclone.
* test/lisp/pcomplete-tests.el (pcomplete-test-parse-gpg-help,
pcomplete-test-parse-git-help): Tests for the new functions.