repmgrd: check binary and extension major versions match

repmgr requires that the same "major version" (e.g. 4.3) is present
on all nodes, otherwise - particularly in the case of repmgrd - it's
highly likely things won't work as expected.

Implements part of GitHub #515.

repmgr_version.h.in

@@ -1,2 +1,3 @@
 #define REPMGR_VERSION_DATE ""
 #define REPMGR_VERSION "4.3dev"
+#define REPMGR_VERSION_NUM 40300