Enable GVL unlocking per default and rename it

Option --disable-gvl-unlock is more understandable.

Benchmarks didn't show a measurable difference between unlocking enabled/disabled, so keeping it enabled is safer.
Should there still be any blocking function calls, GVL unlocking allows to run ruby threads in parallel.

ext/extconf.rb

@@ -15,8 +15,11 @@
 	ENV['PATH'] = "#{pgdir}/bin" + File::PATH_SEPARATOR + ENV['PATH']
 end
 
-if enable_config("nogvl")
-	$defs.push( "-DENABLE_NOGVL" )
+if enable_config("gvl-unlock", true)
+	$defs.push( "-DENABLE_GVL_UNLOCK" )
+	$stderr.puts "Calling libpq with GVL unlocked"
+else
+	$stderr.puts "Calling libpq with GVL locked"
 end
 
 if enable_config("windows-cross")

ext/gvl_wrappers.c

@@ -9,12 +9,12 @@
 char *PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user, const char *algorithm){return NULL;}
 #endif
 
-#ifdef ENABLE_NOGVL
+#ifdef ENABLE_GVL_UNLOCK
 FOR_EACH_BLOCKING_FUNCTION( DEFINE_GVL_WRAPPER_STRUCT );
 FOR_EACH_BLOCKING_FUNCTION( DEFINE_GVL_SKELETON );
 #endif
 FOR_EACH_BLOCKING_FUNCTION( DEFINE_GVL_STUB );
-#ifdef ENABLE_NOGVL
+#ifdef ENABLE_GVL_UNLOCK
 FOR_EACH_CALLBACK_FUNCTION( DEFINE_GVL_WRAPPER_STRUCT );
 FOR_EACH_CALLBACK_FUNCTION( DEFINE_GVLCB_SKELETON );
 #endif

ext/gvl_wrappers.h

@@ -50,7 +50,7 @@
 		return NULL; \
 	}
 
-#ifdef ENABLE_NOGVL
+#ifdef ENABLE_GVL_UNLOCK
 #define DEFINE_GVL_STUB(name, when_non_void, rettype, lastparamtype, lastparamname) \
 	rettype gvl_##name(FOR_EACH_PARAM_OF_##name(DEFINE_PARAM_LIST3) lastparamtype lastparamname){ \
 		struct gvl_wrapper_##name##_params params = { \
@@ -78,7 +78,7 @@
 		return NULL; \
 	}
 
-#ifdef ENABLE_NOGVL
+#ifdef ENABLE_GVL_UNLOCK
 #define DEFINE_GVLCB_STUB(name, when_non_void, rettype, lastparamtype, lastparamname) \
 	rettype gvl_##name(FOR_EACH_PARAM_OF_##name(DEFINE_PARAM_LIST3) lastparamtype lastparamname){ \
 		struct gvl_wrapper_##name##_params params = { \