'cd': properly ignore $CDPATH if initial component is '.' or '..'

@stephane-chazelas writes:
> Per POSIX[*], cd should skip the $CDPATH processing if the first
> component of the directory given to cd is . or ...
>
> Yet, with ksh93u+m 2021-01-03 at least, while that's OK with ..,
> it's not with . with or without the posix option:
>
> $ CDPATH=/ ./ksh -o posix -c 'cd -P ./etc && pwd'
> /etc
> /etc
>
> It seems to be a regression introduced with ksh93u+ as I can't
> reproduce it with ksh93u or any version prior to that. I can also
> reproduce in u+, v- and the ksh2020 from the Ubuntu 20.04
> package.

src/cmd/ksh93/bltins/cd_pwd.c: b_cd():
- Skip $CDPATH processing not only if the path is absolute, but
  also if the initial path component is '.' or '..' (in the latter
  case the $CDPATH processing was done but appeared to be a no-op).

src/cmd/ksh93/tests/builtins.sh:
- Add regression test.

[*] https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/cd.html

Fixes: https://github.com/ksh93/ksh/issues/151

NEWS

@@ -3,6 +3,10 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-01-05:
+
+- Fixed a bug in 'cd' that caused 'cd ./foo' to search for 'foo' in $CDPATH.
+
 2021-01-03:
 
 - The invocation