REORG: vars: move the "proc" scope variables out of the global struct

The presence of this field causes a long dependency chain because almost
everyone includes global-t.h, and vars include sample_data which include
some system includes as well as HTTP parts.

There is absolutely no reason for having the process-wide variables in
the global struct, let's just move them into vars.c and vars.h. This
reduces from ~190k to ~170k the preprocessed output of version.c.
diff --git a/addons/ot/src/vars.c b/addons/ot/src/vars.c
index 0db5514..fff54ac 100644
--- a/addons/ot/src/vars.c
+++ b/addons/ot/src/vars.c
@@ -72,7 +72,7 @@
 	 * source here to get the value of the 'vars' pointer, but it is defined
 	 * as 'static inline', so unfortunately none of this is possible.
 	 */
-	flt_ot_vars_scope_dump(&(global.vars), "PROC");
+	flt_ot_vars_scope_dump(&(proc_vars), "PROC");
 	flt_ot_vars_scope_dump(&(s->sess->vars), "SESS");
 	flt_ot_vars_scope_dump(&(s->vars_txn), "TXN");
 	flt_ot_vars_scope_dump(&(s->vars_reqres), "REQ/RES");
@@ -102,7 +102,7 @@
 	struct vars *retptr = NULL;
 
 	if (strcasecmp(scope, "proc") == 0)
-		retptr = &(global.vars);
+		retptr = &(proc_vars);
 	else if (strcasecmp(scope, "sess") == 0)
 		retptr = (&(s->sess->vars));
 	else if (strcasecmp(scope, "txn") == 0)