Add lots of missing 'static' keywords.

A trawl through the code with -Wmissing-prototypes and
-Wmissing-variable-declarations turned up a lot of things that should
have been internal to a particular source file, but were accidentally
global. Keep the namespace clean by making them all static.

(Also, while I'm here, a couple of them were missing a 'const': the
ONE and ZERO arrays in sshcrcda.c, and EMPTY_WINDOW_TITLE in
terminal.c.)

unix/uxpgnt.c

@@ -45,7 +45,7 @@ static void uxpgnt_log(PageantListenerClient *plc, const char *fmt, va_list ap)
     fprintf(pageant_logfp, "\n");
 }
 
-const PageantListenerClientVtable uxpgnt_vtable = {
+static const PageantListenerClientVtable uxpgnt_vtable = {
     uxpgnt_log,
 };