Allow turning off brace expansion in comsubs (rhbz#1078698)

There was no check for the -B/braceexpand option before calling
path_expand() to process brace expansion, making it impossible to
turn off brace expansion within command substitutions. Normally the
lexer flags brace expansion so that this code is not reached, but
shell code within command substitutions is handled differently.

Red Hat patches this by adding this check to the function itself:
642af4d6/f/ksh-20140301-fikspand.patch
But I think it's more logical to patch it at the point of decision.

src/cmd/ksh93/sh/macro.c: endfield():
- Decide to call either path_generate() or path_expand() based on
  the state of the SH_BRACEEXPAND shell option.
- Fix '#if SHOPT_BRACEPAT' preprocessor check that previously
  hardcoded this decision at compile time.

src/cmd/ksh93/tests/options.sh:
- Add tests.

NEWS

@@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-24:
+
+- An omission made it impossible to turn off brace expansion within command
+  substitutions (`...`, $(...) or ${ ...; }) as the code for parsing these
+  did not check the -B/braceexpand option. This check has now been added.
+
 2020-09-23:
 
 - Fixed a crash that could occur when running a pipeline containing