subprocess: check and handle fcntl(F_GETFD) failure

Add missing error check for fcntl(fd, F_GETFD, 0) in set_clo_on_exec.
Raise OSError on failure to align with existing FD_SETFD behavior.
This improves robustness in subprocess setup and error visibility.

Github-Pull: arun11299/cpp-subprocess#117
Rebased-From: 9974ff69cdd5fc1a2722cb63f006df9308628b35

Author: tomasandroil

src/util/subprocess.h

@@ -346,10 +346,14 @@ namespace util
   void set_clo_on_exec(int fd, bool set = true)
   {
     int flags = fcntl(fd, F_GETFD, 0);
+    if (flags == -1) {
+        throw OSError("fcntl F_GETFD failed", errno);
+    }
     if (set) flags |= FD_CLOEXEC;
     else flags &= ~FD_CLOEXEC;
-    //TODO: should check for errors
-    fcntl(fd, F_SETFD, flags);
+    if (fcntl(fd, F_SETFD, flags) == -1) {
+        throw OSError("fcntl F_SETFD failed", errno);
+    }
   }