PostgreSQL 18 support

[tbussmann]

Error when building against the git master of PostgreSQL:

plpgsql_debugger.c:685:61: error: no member named 'attrs' in 'struct TupleDescData'
                dbg_send( "v:%s.%s:%s\n", var_name, NameStr( rec_tupdesc->attrs[attNo].attname ), extval ? extval : "NULL" );
                                                             ~~~~~~~~~~~  ^
/Applications/Postgres.app/Contents/Versions/devel/include/postgresql/server/c.h:703:25: note: expanded from macro 'NameStr'
#define NameStr(name)   ((name).data)
                          ^~~~

This apparently needs adjustments to deal with 5983a4cff like injecting a call to TupleDescAttr to retrieve attname.