gssapi: rename our files to avoid conflicts

gssapi.h is used as a header name by Heimdal-style GSSAPI so it would
conflict with a private header using that name, and while renaming the
header I figured we should name the .c file accordingly as well.

Bug: http://curl.haxx.se/mail/lib-2011-07/0071.html
Reported by: Ben Greear

lib/curl_gssapi.h

@@ -0,0 +1,47 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at http://curl.haxx.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+
+#include "setup.h"
+
+#ifdef HAVE_GSSGNU
+#  include <gss.h>
+#elif defined HAVE_GSSMIT
+   /* MIT style */
+#  include <gssapi/gssapi.h>
+#  include <gssapi/gssapi_generic.h>
+#  include <gssapi/gssapi_krb5.h>
+#else
+   /* Heimdal-style */
+#  include <gssapi.h>
+#endif
+
+
+/* Common method for using gss api */
+
+OM_uint32 Curl_gss_init_sec_context(
+    OM_uint32 * minor_status,
+    gss_ctx_id_t * context,
+    gss_name_t target_name,
+    gss_channel_bindings_t input_chan_bindings,
+    gss_buffer_t input_token,
+    gss_buffer_t output_token,
+    OM_uint32 * ret_flags);