Fix more spurious comsub execution in tab completion (re: 7a2d3564)

Comsubs were either executed or caused a syntax error when attempting
to complete them within single quotes. Since single quotes do not
expand anything, no such completion should take place.

$ '`/de<TAB>-ksh: /dev/: cannot execute [Is a directory]

$ '$(/de<TAB>-ksh: syntax error: `end of file' unexpected

src/cmd/ksh93/edit/completion.c:
- find_begin():
  - Remove recursive handling for '`' comsubs from 7a2d3564; it is
    sufficient to set the return pointer to the current location cp
    (the character following '`') if we're not in single quotes.
  - For '$' and '`', if we are within single quotes, set type '\''
    and set the return pointer bp to the location of the '$' or
    '`'.
- ed_expand(): If find_begin() sets type '\'' and the current begin
  character is $ or `, refuse to attempt completion; return -1 to
  cause a terminal beep.

Related:
https://github.com/ksh93/ksh/issues/268
https://github.com/ksh93/ksh/issues/462#issuecomment-1038482307

NEWS

@@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-06-05:
+
+- Fixed a bug where tab completion would spuriously execute a command
+  substitution after entering: '`something<TAB> or generate a spurious
+  'end of file unexpected' syntax error after entering: '$(something<TAB>.
+
 2022-06-04:
 
 - Added a new --functrace long-form shell option which causes the -x/--xtrace