relicense from (GPLv3) to (LGPLv2.1 or later)

Relicense from (GPLv3) to (LGPLv2.1 or later):

  * Change existing license headers from (GPLv3) to (LGPLv2.1 or later)
  * Add (LGPLv2.1 or later) licensing headers to Makefile, to example XML
    files, to examples/python-src-example.c and to firehose.rng
  * Replace COPYING with lgpl-2.1.txt
  * Update license metadata in setup.py
  * Update line numbers in the example XML files that refer to
    python-src-example.c to reflect the new line numbering due to the
    license header