Add "e" flag to fopen() calls

This adds the `e` flag to fopen() calls, making sure the `O_CLOEXEC` flag is
used. This makes sure that the file descriptor is being closed and not leaked
into child processes. This was an issues previously due to a missing fclose()
(Yubico#136).

pam_yubico.c

@@ -819,7 +819,7 @@ parse_cfg (int flags, int argc, const char **argv, struct cfg *cfg)
                 {
                   if(S_ISREG(st.st_mode))
                     {
-                      file = fopen(filename, "a");
+                      file = fopen(filename, "ae");
                       if(file)
                         {
                           cfg->debug_file = file;

util.c

@@ -188,7 +188,7 @@ int generate_random(void *buf, int len)
 	FILE *u;
 	int res;
 
-	u = fopen("/dev/urandom", "r");
+	u = fopen("/dev/urandom", "re");
 	if (!u) {
 		return -1;
 	}

ykpamcfg.c

@@ -237,7 +237,7 @@ do_add_hmac_chalresp(YK_KEY *yk, uint8_t slot, bool verbose, char *output_dir, u
 
   umask(077);
 
-  f = fopen (fn, "w");
+  f = fopen (fn, "we");
   if (! f) {
     fprintf (stderr, "Failed opening '%s' for writing : %s\n", fn, strerror (errno));
     goto out;