Revert "fcntl a little less"

This reverts commits:
e5362a4ae5.
dd9a26715d.

These commits were incorrect because they stomped other flags, such as
O_NONBLOCK.

src/fish.cpp

@@ -441,14 +441,13 @@ int main(int argc, char **argv) {
             res = reader_read(STDIN_FILENO, {});
         } else {
             char *file = *(argv + (my_optind++));
-#if defined(O_CLOEXEC)
-            int fd = open(file, O_RDONLY | O_CLOEXEC);
-#else
             int fd = open(file, O_RDONLY);
-#endif
             if (fd == -1) {
                 perror(file);
             } else {
+                // OK to not do this atomically since we cannot have gone multithreaded yet.
+                set_cloexec(fd);
+
                 wcstring_list_t list;
                 for (char **ptr = argv + my_optind; *ptr; ptr++) {
                     list.push_back(str2wcstring(*ptr));

src/wutil.cpp

@@ -176,9 +176,16 @@ FILE *wfopen(const wcstring &path, const char *mode) {
 }
 
 bool set_cloexec(int fd) {
-    int flags = fcntl(fd, F_SETFD, FD_CLOEXEC);
-    if (flags == -1) return false;
-    return true;
+    // Note we don't want to overwrite existing flags like O_NONBLOCK which may be set. So fetch the
+    // existing flags and OR in our new one.
+    int flags = fcntl(fd, F_GETFD, 0);
+    if (flags < 0) {
+        return false;
+    }
+    if (flags & FD_CLOEXEC) {
+        return true;
+    }
+    return fcntl(fd, F_SETFD, flags | FD_CLOEXEC) >= 0;
 }
 
 static int wopen_internal(const wcstring &pathname, int flags, mode_t mode, bool cloexec) {