netrc: fixed thread safety problem by using getpwuid_r if available

The old way using getpwuid could cause problems in programs that enable
reading from netrc files simultaneously in multiple threads.

Reported-by: David Woodhouse

lib/netrc.c

@@ -76,7 +76,19 @@ int Curl_parsenetrc(const char *host,
     char *home = curl_getenv("HOME"); /* portable environment reader */
     if(home) {
       home_alloc = TRUE;
-#if defined(HAVE_GETPWUID) && defined(HAVE_GETEUID)
+#if defined(HAVE_GETPWUID_R) && defined(HAVE_GETEUID)
+    }
+    else {
+      struct passwd pw, *pw_res;
+      char pwbuf[1024];
+      if(!getpwuid_r(geteuid(), &pw, pwbuf, sizeof(pwbuf), &pw_res)
+         && pw_res) {
+        home = strdup(pw.pw_dir);
+        if(!home)
+          return CURLE_OUT_OF_MEMORY;
+        home_alloc = TRUE;
+      }
+#elif defined(HAVE_GETPWUID) && defined(HAVE_GETEUID)
     }
     else {
       struct passwd *pw;