Add the '-e' flag to the 'cd' builtin (#358)

This change adds the -e flag to the cd builtin, as specified in
<https://www.austingroupbugs.net/view.php?id=253>. The -e flag is
used to verify if the the current working directory after 'cd -P'
successfully changes the directory, and returns with exit status 1
if the cwd couldn't be determined. Additionally, it causes all
other errors to return with exit status >1 (i.e., status 2 unless
ENOMEM occurs) if -e and -P are both active.

src/cmd/ksh93/bltins/cd_pwd.c:
- Add -e option to the cd builtin command. It verifies $PWD by
  using test_inode() to execute the equivalent of [[ . -ef $PWD ]].
- The check for restricted mode has been moved after optget to
  allow 'cd -eP' to return with exit status 2 when in restricted
  mode. To avoid changing the previous behavior of cd when -e isn't
  passed, extra checks have been added to prevent cd from printing
  usage information in restricted mode.

src/cmd/ksh93/tests/builtins.sh:
- Add regression tests for the exit status when using the cd -P
  flag with and without -e.

src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/sh.1:
- Document the addition of -e to the cd builtin.

NEWS

@@ -8,6 +8,12 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Fixed an issue on illumos that caused some parameters in the getconf
   builtin to fail.
 
+- The cd built-in command now supports a -e option (as specified in
+  https://www.austingroupbugs.net/view.php?id=253). Passing -e alongside -P
+  is used to guarantee the cd built-in returns with exit status 1 if the
+  current working directory couldn't be determined after successfully changing
+  the directory.
+
 2021-12-01:
 
 - Fixed a memory fault that occurred when a discipline function exited