clone.2: Use pid_t for clone3() {child,parent}_tid

Advertise to userspace that they should use proper pid_t types
for arguments returning a pid.

The kernel-internal struct kernel_clone_args currently uses int
as type and since POSIX mandates that pid_t is a signed integer
type and glibc and friends use int this is not an issue. After
the merge window for v5.5 closes we can switch struct
kernel_clone_args over to using pid_t as well without any danger
in regressing current userspace.

Also note, that the new set tid feature which will be merged for
v5.5 uses pid_t types as well.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/clone.2

@@ -183,9 +183,9 @@ is a structure of the following form:
 struct clone_args {
     u64 flags;        /* Flags bit mask */
     u64 pidfd;        /* Where to store PID file descriptor
-                         (\fIint *\fP) */
+                         (\fIpid_t *\fP) */
     u64 child_tid;    /* Where to store child TID,
-                         in child's memory (\fIint *\fP) */
+                         in child's memory (\fIpid_t *\fP) */
     u64 parent_tid;   /* Where to store child TID,
                          in parent's memory (\fIint *\fP) */
     u64 exit_signal;  /* Signal to deliver to parent on