lib: make some structures constant

* isisd/isis_pdu.c
  * maskbit: this pre-initialized array is not modified
* lib/prefix.c
  * maskbit: idem
* lib/command.c
  * facility_map: idem
  * itoa64: idem
  * default_motd: make local var static
  * facility_name(): update local var accordingly
  * facility_match(): idem
diff --git a/lib/command.c b/lib/command.c
index 31c067a..478125f 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -82,14 +82,14 @@
 };
 
 /* Default motd string. */
-const char *default_motd =
+static const char *default_motd =
 "\r\n\
 Hello, this is " QUAGGA_PROGNAME " (version " QUAGGA_VERSION ").\r\n\
 " QUAGGA_COPYRIGHT "\r\n\
 \r\n";
 
 
-static struct facility_map {
+static const struct facility_map {
   int facility;
   const char *name;
   size_t match;
@@ -122,7 +122,7 @@
 static const char *
 facility_name(int facility)
 {
-  struct facility_map *fm;
+  const struct facility_map *fm;
 
   for (fm = syslog_facilities; fm->name; fm++)
     if (fm->facility == facility)
@@ -133,7 +133,7 @@
 static int
 facility_match(const char *str)
 {
-  struct facility_map *fm;
+  const struct facility_map *fm;
 
   for (fm = syslog_facilities; fm->name; fm++)
     if (!strncmp(str,fm->name,fm->match))
@@ -506,7 +506,7 @@
   cmd->cmdsize = cmd_cmdsize (cmd->strvec);
 }
 
-static unsigned char itoa64[] =	
+static const unsigned char itoa64[] =
 "./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
 
 static void