Define P_PIDFD, used with waitid

Linux defines a waitid type `P_PIDFD`, for use with process file
descriptors (`pidfd`). Add that constant.

In libc-test, add linux/wait.h to the Linux-specific and
Android-specific headers, to get the definition. Exclude it on Android
and musl for now, though, as the versions in CI don't have it yet.

Author: joshtriplett

libc-test/build.rs

@@ -1486,6 +1486,7 @@ fn test_android(target: &str) {
                 "linux/seccomp.h",
                 "linux/sched.h",
                 "linux/sockios.h",
+                "linux/wait.h",
 
     }
 
@@ -1556,6 +1557,9 @@ fn test_android(target: &str) {
             // FIXME: deprecated - removed in glibc 2.26
             "SIGUNUSED" => true,
 
+            // Needs a newer Android SDK for the definition
+            "P_PIDFD" => true,
+
             _ => false,
         }
     });
@@ -2442,6 +2446,7 @@ fn test_linux(target: &str) {
         "linux/seccomp.h",
         "linux/sockios.h",
         "linux/vm_sockets.h",
+        "linux/wait.h",
         "sys/auxv.h",
         "sys/fanotify.h",
     }
@@ -2626,6 +2631,10 @@ fn test_linux(target: &str) {
             // Require Linux kernel 5.6:
             "VMADDR_CID_LOCAL" => true,
 
+            // Defined in kernel headers but musl removes it; need musl 1.2 for definition in musl
+            // headers.
+            "P_PIDFD" => true,
+
             _ => false,
         }
     });

src/unix/linux_like/mod.rs

@@ -1034,6 +1034,11 @@ pub const SIGEV_THREAD: ::c_int = 2;
 pub const P_ALL: idtype_t = 0;
 pub const P_PID: idtype_t = 1;
 pub const P_PGID: idtype_t = 2;
+cfg_if! {
+    if #[cfg(not(target_os = "emscripten"))] {
+        pub const P_PIDFD: idtype_t = 3;
+    }
+}
 
 pub const UTIME_OMIT: c_long = 1073741822;
 pub const UTIME_NOW: c_long = 1073741823;