Fix bugs, inefficiencies and bad style in regexps

Fix bugs, inefficiencies and bad style in regexps

Found by relint.  See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2020-04/msg00265.html

* lisp/org/org-table.el (org-table-finish-edit-field):
* lisp/arc-mode.el (archive-rar-summarize):
Avoid wrapped subsumption in repeated sequences.
* lisp/erc/erc-dcc.el (erc-dcc-ctcp-query-send-regexp): Replace
inefficient repeated empty-matching expression with a plain greedy
form.
(erc-dcc-handle-ctcp-send): Adjust group numbers.
* lisp/net/puny.el (puny-encode-domain): Fix fast-path shortcut
pattern so that it actually works as intended.
* lisp/progmodes/gdb-mi.el (gdb-control-commands-regexp):
* lisp/vc/diff-mode.el (diff-imenu-generic-expression):
Remove superfluous backslashes.
* lisp/progmodes/scheme.el (scheme-imenu-generic-expression):
Correct confused definition-matching pattern which would match more
than intended.
* lisp/textmodes/sgml-mode.el (sgml-tag-name-re): Avoid inefficient
matching by using the fact that the first character cannot match the
last char of sgml-name-re.