Remove WNOHANG flag from wait3().

Remove WNOHANG flag from wait3().

Because script(1) now reliably terminates when the TTY is closed, it may
be the case that the call to wait3() occurs just before the child
process exits. This causes error codes to be ignored.

Just change script(1) to use waitpid() instead of wait3(). This makes it
more portable and prevents the need for a loop, since waitpid() only
returns a specified process.

PR:		bin/146189
Tested by:	amdmi3@, older version
MFC after:	2 weeks