erlxc: compact "-v" switch
tests: optional verbose mode
Make the verbose levels a bit more consistent
tests: optionally run with valgrind

Allow overriding the exec parameter in tests for use with valgrind:

    ERLXC_TEST_EXEC="sudo valgrind --leak-check=yes --log-file=/tmp/erlxc.log" make test

valgrind must be set up in sudoers.
Fix memory leak in argv