Add primary_node_info inline function

When a t_node_info struct must be re initilised, it was re-declared
only. Moreover, a macro was user for any var creation of this type
impeding use the same to reset the values later if needed.

This new function allows to re initilise again a t_node_info typed
variable without the need to redeclare it to create other varibles for
the same purpose, and also shadowing is now avoided from pg16.

Macros seems to be replaced by `static inline` functions in upstream
postgres, credits to  Alvaro Herrera <alvherre@alvh.no-ip.org> for this
idea.

References: HL-40

repmgr.control

@@ -1,6 +1,6 @@
 # repmgr extension
 comment = 'Replication manager for PostgreSQL'
-default_version = '5.5'
+default_version = '5.4'
 module_pathname = '$libdir/repmgr'
 relocatable = false
 schema = repmgr