[XrdHttp] Fix time-of-check vs time-of-use race condition

First open then fstat, instead of stat then fopen, to guarantee that
the information from stat and open are consistent with each other.
Afterwards, convert file descriptor to FILE* with fdopen. Ensure also
that the file is closed should an error occur.

src/XrdHttp/XrdHttpProtocol.cc

@@ -2235,20 +2235,31 @@ int XrdHttpProtocol::xsecretkey(XrdOucStream & Config) {
   if (val[0] == '/') {
     struct stat st;
     inFile = true;
-    if ( stat(val, &st) ) {
-      eDest.Emsg("Config", errno, "stat shared secret key file", val);
+    int fd = open(val, O_RDONLY);
+
+    if ( fd == -1 ) {
+      eDest.Emsg("Config", errno, "open shared secret key file", val);
+      return 1;
+    }
+
+    if ( fstat(fd, &st) != 0 ) {
+      eDest.Emsg("Config", errno, "fstat shared secret key file", val);
+      close(fd);
       return 1;
     }
 
     if ( st.st_mode & S_IWOTH & S_IWGRP & S_IROTH) {
-      eDest.Emsg("Config", "For your own security, the shared secret key file cannot be world readable or group writable'", val, "'");
+      eDest.Emsg("Config",
+        "For your own security, the shared secret key file cannot be world readable or group writable '", val, "'");
+      close(fd);
       return 1;
     }
 
-    FILE *fp = fopen(val,"r");
+    FILE *fp = fdopen(fd, "r");
 
-    if( fp == NULL ) {
-      eDest.Emsg("Config", errno, "open shared secret key file", val);
+    if ( fp == nullptr ) {
+      eDest.Emsg("Config", errno, "fdopen shared secret key file", val);
+      close(fd);
       return 1;
     }