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>

ps/stacktrace.c

@@ -1,8 +1,23 @@
 /*
+ * stacktrace.c - ps debugging additions
  * Gnu debugger stack trace code provided by Peter Mattis
  * <petm@CSUA.Berkeley.EDU> on Thu, 2 Nov 1995
  *
  * Modified for easy use by Albert Cahalan.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
 #include <errno.h>