linux: Doc and check retval for no-fail pid calls

The switch from @bitcast() to @intcast() here safety-checks
Linux's assertion that these 3 calls never return errors (negative
values as pid_t).  getppid() can legally return 0 if the parent is
in a different pid namespace, but this is not an error.

Author: blblack

lib/std/os/linux.zig

@@ -1839,15 +1839,18 @@ pub fn setsid() usize {
 }
 
 pub fn getpid() pid_t {
-    return @bitCast(@as(u32, @truncate(syscall0(.getpid))));
+    // Casts result to a pid_t, safety-checking >= 0, because getpid() cannot fail
+    return @intCast(@as(u32, @truncate(syscall0(.getpid))));
 }
 
 pub fn getppid() pid_t {
-    return @bitCast(@as(u32, @truncate(syscall0(.getppid))));
+    // Casts result to a pid_t, safety-checking >= 0, because getppid() cannot fail
+    return @intCast(@as(u32, @truncate(syscall0(.getppid))));
 }
 
 pub fn gettid() pid_t {
-    return @bitCast(@as(u32, @truncate(syscall0(.gettid))));
+    // Casts result to a pid_t, safety-checking >= 0, because gettid() cannot fail
+    return @intCast(@as(u32, @truncate(syscall0(.gettid))));
 }
 
 pub fn sigprocmask(flags: u32, noalias set: ?*const sigset_t, noalias oldset: ?*sigset_t) usize {