docs: clarification to license headers in files

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>