Fix handling of SIGCHLD/sigaction.

Use gnulib's sigaction module to get sigaction on every platform.
However, some platforms (mingw specifically) lack SIGCHLD, so change
ifdefs to be on SIGCHLD, not HAVE_SIGACTION.

Remove the nobackground instruction, as blocking SIGTSTP is not cw's
job.
sscanf and isxdigit are C89 functions, so assume them.