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.)

windows/winpgnt.c

@@ -1188,7 +1188,7 @@ int flags = FLAG_SYNCAGENT;
 struct winpgnt_client {
     PageantListenerClient plc;
 };
-const PageantListenerClientVtable winpgnt_vtable = {
+static const PageantListenerClientVtable winpgnt_vtable = {
     NULL, /* no logging */
 };
 

windows/winpgntc.c

@@ -210,7 +210,7 @@ static size_t named_pipe_agent_gotdata(
     return 0;
 }
 
-agent_pending_query *named_pipe_agent_query(
+static agent_pending_query *named_pipe_agent_query(
     strbuf *query, void **out, int *outlen,
     void (*callback)(void *, void *, int), void *callback_ctx)
 {

windows/winsecur.c

@@ -92,7 +92,7 @@ PSID get_user_sid(void)
     return ret;
 }
 
-bool getsids(char **error)
+static bool getsids(char **error)
 {
 #ifdef __clang__
 #pragma clang diagnostic push