Add pxd for wait & friends from "sys/wait.h" covering POSIX/general Unix.

Cython/Includes/posix/wait.pxd

+# http://pubs.opengroup.org/onlinepubs/009695399/basedefs/sys/wait.h.html
+
+from posix.types cimport pid_t, id_t
+from posix.signal cimport siginfo_t
+from posix.resource cimport rusage
+
+cdef extern from "sys/wait.h" nogil:
+    enum: WNOHANG
+    enum: WUNTRACED
+    enum: WCONTINUED
+    enum: WEXITED
+    enum: WSTOPPED
+    enum: WNOWAIT
+
+    int WEXITSTATUS(int status)
+    int WIFCONTINUED(int status)
+    int WIFEXITED(int status)
+    int WIFSIGNALED(int status)
+    int WIFSTOPPED(int status)
+    int WSTOPSIG(int status)
+    int WTERMSIG(int status)
+
+    ctypedef int idtype_t
+    enum: P_ALL             # idtype_t values
+    enum: P_PID
+    enum: P_PGID
+
+    pid_t wait(int *stat_loc)
+    pid_t waitpid(pid_t pid, int *status, int options)
+    int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options)
+
+# wait3 was in POSIX until 2008 while wait4 was never standardized.
+# Even so, these calls are in almost every Unix, always in sys/wait.h.
+# Hence, posix.wait is the least surprising place to declare them for Cython.
+# libc may require _XXX_SOURCE to be defined at C-compile time to provide them.
+
+    pid_t wait3(int *status, int options, rusage *rusage)
+    pid_t wait4(pid_t pid, int *status, int options, rusage *rusage)