[test suite] added coverage_diff.py

This commit adds support for diff'ing before and after gcov/lcov results
to see when new function/line coverage is added by the test suite.  Here
is an example of its output:

Sun Jun  1 22:28:00 2014 CMD: ./coverage_diff.py
[+] Coverage: /home/mbr/git/fwknop.git/server/config_init.c
[+] new 'fcns' coverage: usage()
[+] new 'lines' coverage: 1015
[+] new 'lines' coverage: 1017
[+] new 'lines' coverage: 1019
[+] new 'lines' coverage: 1059
[+] new 'lines' coverage: 979
[+] Coverage: /home/mbr/git/fwknop.git/server/fw_util_iptables.c
[+] new 'lines' coverage: 560
[+] new 'lines' coverage: 561
add --fault-injection-tag support to the client/server/libfko

This is a significant commit to add the ability to leverage libfko fault
injections from both the fwknop client and server command lines via a
new option '--fault-injection-tag <tag name>'.  This option is used by
the test suite with the tests/fault_injection.pl tests.