docs: clarification to license headers in files

Add license header to all files.  The summary of licensing is below,
taken from Craig Small's email which is referred in commit message
tail.

sysctl and pgrep are GPL 2+
The rest is LGPL 2.1+

Reference: http://www.freelists.org/post/procps/Incorrect-FSF-address-in-the-license-files,8
Bug-Redhat: https://bugzilla.redhat.com/show_bug.cgi?id=797962
CC: Craig Small <csmall@enc.com.au>
CC: Jaromir Capik <jcapik@redhat.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

free.c

@@ -2,14 +2,28 @@
  * free.c - free(1)
  * procps-ng utility to display free memory information
  *
+ * Copyright (C) 1992-2012
+ *
  * Mostly new, Sami Kerola <kerolasa@iki.fi>		15 Apr 2011
  * All new, Robert Love <rml@tech9.net>			18 Nov 2002
  * Original by Brian Edmonds and Rafal Maszkowski	14 Dec 1992
  *
- * This program is licensed under the GNU Library General Public License, v2
- *
  * Copyright 2003 Robert Love
  * Copyright 2004 Albert Cahalan
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  */
 
 #include "config.h"