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/Makefile.inc

@@ -22,7 +22,7 @@ CSOURCES = file.c timeval.c base64.c hostip.c progress.c formdata.c	\
   pingpong.c rtsp.c curl_threads.c warnless.c hmac.c polarssl.c		\
   curl_rtmp.c openldap.c curl_gethostname.c gopher.c axtls.c		\
   idn_win32.c http_negotiate_sspi.c cyassl.c http_proxy.c non-ascii.c	\
-  asyn-ares.c asyn-thread.c gssapi.c
+  asyn-ares.c asyn-thread.c curl_gssapi.c
 
 HHEADERS = arpa_telnet.h netrc.h file.h timeval.h qssl.h hostip.h	\
   progress.h formdata.h cookie.h http.h sendf.h ftp.h url.h dict.h	\
@@ -37,4 +37,4 @@ HHEADERS = arpa_telnet.h netrc.h file.h timeval.h qssl.h hostip.h	\
   curl_base64.h rawstr.h curl_addrinfo.h curl_sspi.h slist.h nonblock.h	\
   curl_memrchr.h imap.h pop3.h smtp.h pingpong.h rtsp.h curl_threads.h	\
   warnless.h curl_hmac.h polarssl.h curl_rtmp.h curl_gethostname.h	\
-  gopher.h axtls.h cyassl.h http_proxy.h non-ascii.h asyn.h gssapi.h
+  gopher.h axtls.h cyassl.h http_proxy.h non-ascii.h asyn.h curl_gssapi.h

lib/curl_gssapi.c

@@ -20,7 +20,7 @@
  *
  ***************************************************************************/
 
-#include "gssapi.h"
+#include "curl_gssapi.h"
 
 OM_uint32 Curl_gss_init_sec_context(
     OM_uint32 * minor_status,

lib/http_negotiate.c

@@ -36,7 +36,7 @@
 
 #include "urldata.h"
 #include "sendf.h"
-#include "gssapi.h"
+#include "curl_gssapi.h"
 #include "rawstr.h"
 #include "curl_base64.h"
 #include "http_negotiate.h"

lib/krb5.c

@@ -50,7 +50,7 @@
 #include "urldata.h"
 #include "curl_base64.h"
 #include "ftp.h"
-#include "gssapi.h"
+#include "curl_gssapi.h"
 #include "sendf.h"
 #include "krb4.h"
 #include "curl_memory.h"

lib/socks_gssapi.c

@@ -37,7 +37,7 @@
 #include <stdlib.h>
 #endif
 
-#include "gssapi.h"
+#include "curl_gssapi.h"
 #include "urldata.h"
 #include "sendf.h"
 #include "connect.h"