Bug Fixes
---------

- Fix CPU_SET(3) macro type mismatch by making the macros type-safe.
  Addresses https://cygwin.com/pipermail/cygwin/2023-March/253220.html

- kill(1): don't print spurious error message.
  Addresses: https://cygwin.com/pipermail/cygwin/2023-March/253291.html

- Align behaviour of dirname in terms of leading slashes to POSIX:
  https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html