Create and use struct_sysent and struct_ioctlent typedefs.

This is a preparatory mass replace patch with no code changes.

The future change will need to typedef sysent to sysent0,
which results in compile failures when "struct sysent" string
gets mangled into "struct sysent0".

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/pathtrace.c b/pathtrace.c
index 7f90dd5..f202c99 100644
--- a/pathtrace.c
+++ b/pathtrace.c
@@ -157,7 +157,7 @@
 int
 pathtrace_match(struct tcb *tcp)
 {
-	const struct sysent *s;
+	const struct_sysent *s;
 
 	if (selected[0] == NULL)
 		return 1;