'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

src/cmd/ksh93/tests/builtins.sh

@@ -832,6 +832,12 @@ actual=$({ a=$(cd; pwd); } >&-; print -r -- "$a")
 [[ $actual == "$expect" ]] || err_exit "'cd' broke new-form command substitution with outer stdout closed" \
 	"(expected $(printf %q "$expect"), got $(printf %q "$actual"))"
 
+# CDPATH was not ignored by 'cd ./dir': https://github.com/ksh93/ksh/issues/151
+expect=': cd: ./dev: [No such file or directory]'
+actual=$( (CDPATH=/ cd -P ./dev && pwd) 2>&1 )
+let "(e=$?)==1" && [[ $actual == *"$expect" ]] || err_exit "CDPATH not ignored by cd ./dir" \
+	"(expected *$(printf %q "$expect") with status 1, got $(printf %q "$actual") with status $e)"
+
 # ======
 # 'readonly' should set the correct scope when creating variables in functions
 unset foo