Avoid using `#include_next` GNU extension ...

[johnsonjh]

• #include_next is non-POSIX (a GNU extension) and is currently used in:
  • include/bsd_stdlib.h
  • include/bsd_string.h
  • include/sys/types.h
  • include/sys/time.h
  • include/sys/stat.h
  • include/bsd_termios.h
  • include/bsd_fcntl.h
  • include/bsd_unistd.h
  • include/libgen.h
  • include/poll.h