Ticket #4050: code cleanup before 4.8.25 release.

Update copyright years.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

lib/charsets.c

@@ -1,7 +1,7 @@
 /*
    Text conversion from one charset to another.
 
-   Copyright (C) 2001-2019
+   Copyright (C) 2001-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/event/event.c

@@ -2,7 +2,7 @@
    Handle  events in application.
    Interface functions: init/deinit; start/stop
 
-   Copyright (C) 2011-2019
+   Copyright (C) 2011-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/event/manage.c

@@ -2,7 +2,7 @@
    Handle any events in application.
    Manage events: add, delete, destroy, search
 
-   Copyright (C) 2011-2019
+   Copyright (C) 2011-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/event/raise.c

@@ -2,7 +2,7 @@
    Handle any events in application.
    Raise events.
 
-   Copyright (C) 2011-2019
+   Copyright (C) 2011-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/filehighlight/common.c

@@ -2,7 +2,7 @@
    File highlight plugin.
    Interface functions
 
-   Copyright (C) 2009-2019
+   Copyright (C) 2009-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/filehighlight/get-color.c

@@ -2,7 +2,7 @@
    File highlight plugin.
    Interface functions. get color pair index for highlighted file.
 
-   Copyright (C) 2009-2019
+   Copyright (C) 2009-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/filehighlight/ini-file-read.c

@@ -2,7 +2,7 @@
    File highlight plugin.
    Reading and parse rules from ini-files
 
-   Copyright (C) 2009-2019
+   Copyright (C) 2009-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/glibcompat.c

@@ -1,7 +1,7 @@
 /*
    GLIB - Library of useful routines for C programming
 
-   Copyright (C) 2009-2019
+   Copyright (C) 2009-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/global.c

@@ -1,7 +1,7 @@
 /*
    Global structure for some library-related variables
 
-   Copyright (C) 2009-2019
+   Copyright (C) 2009-2020
    Free Software Foundation, Inc.
 
    Written by:

lib/hook.c

@@ -4,7 +4,7 @@
    Slavaz: Warning! this file is deprecated and should be replaced
    by mcevents functional.
 
-   Copyright (C) 1994-2019
+   Copyright (C) 1994-2020
    Free Software Foundation, Inc.
 
    Written by: