Return ID of new session in setsid.

src/platform/linux/mod.rs

@@ -612,8 +612,8 @@ impl Pal for Sys {
         e_raw(unsafe { syscall!(SETRESUID, ruid, euid, suid) }).map(|_| ())
     }
 
-    fn setsid() -> Result<()> {
-        e_raw(unsafe { syscall!(SETSID) }).map(|_| ())
+    fn setsid() -> Result<c_int> {
+        e_raw(unsafe { syscall!(SETSID) }).map(|s| s as c_int)
     }
 
     fn symlink(path1: CStr, path2: CStr) -> Result<()> {

src/platform/pal/mod.rs

@@ -237,7 +237,7 @@ pub trait Pal {
 
     fn setresuid(ruid: uid_t, euid: uid_t, suid: uid_t) -> Result<()>;
 
-    fn setsid() -> Result<()>;
+    fn setsid() -> Result<c_int>;
 
     fn symlink(path1: CStr, path2: CStr) -> Result<()>;
 

src/platform/redox/mod.rs

@@ -890,9 +890,8 @@ impl Pal for Sys {
         Err(Errno(ENOSYS))
     }
 
-    fn setsid() -> Result<()> {
-        redox_rt::sys::posix_setsid()?;
-        Ok(())
+    fn setsid() -> Result<c_int> {
+        Ok(redox_rt::sys::posix_setsid()? as c_int)
     }
 
     fn setresgid(rgid: gid_t, egid: gid_t, sgid: gid_t) -> Result<()> {