Enable user-defined actions

Also fix pclose() handling within Info plugins,
and prevent them from screwing up the terminal with error output
on initialization.

This is still rather crude, but at least it's possible.

nncmpp.actions.awk

@@ -1,6 +1,6 @@
 # nncmpp.actions.awk: produce C code for a list of user actions
 #
-# Copyright (c) 2022, Přemysl Eric Janouch <p@janouch.name>
+# Copyright (c) 2022 - 2024, Přemysl Eric Janouch <p@janouch.name>
 # SPDX-License-Identifier: 0BSD
 #
 # Usage: env LC_ALL=C A=0 B=1 awk -f nncmpp.actions.awk \
@@ -91,7 +91,7 @@ END {
 	print "enum action {"
 	for (i in Constants)
 		print "\t" "ACTION_" Constants[i] ","
-	print "\t" "ACTION_COUNT"
+	print "\t" "ACTION_USER_0"
 	print "};"
 	print ""
 	print "static const char *g_action_names[] = {"