* posix/sys/wait.h [__WIFCONTINUED] (WIFCONTINUED): New macro.
* stdlib/stdlib.h [__WIFCONTINUED] (WIFCONTINUED): New macro.
* sysdeps/generic/bits/waitstatus.h (__W_CONTINUED): New macro.
[WCONTINUED] (__WIFCONTINUED): New macro.
(__WIFSIGNALED): Rewritten to exclude __W_CONTINUED value, and have no
branches.