Upgrade net/axa to version 1.6.0:

• Fix make check Clang compiler warnings
• Fix small bugs in apikey transport
• Fix sratool/radtool command completion bug
• Fix sratool/radtool overzealous help bug
• Fix make check when building without yajl
• Add axa_apikey_load_and_check_key()
• Update README (additional examples)
• Remove configure option to select compile-time list of available ciphers
• Make config file errors emitted according to axa_debug level
• Make nmsg file and udp output objects unbuffered

Sponsored by: Farsight Security, Inc.