Logging changes

Continuation of https://github.com/janetuk/trust_router/pull/4, taking into account feedback from Sam.

Important notes:
 * gsscon is unaffected by this change - there's a handful of printf's left in there as it is messy to correctly import the files needed as it is compiled as a separate library.
 * Severities defined in the log file are lower case analogues of syslog severity levels, as strcmp()'ing macro names is messy.
 * Probably some improvements to be made with classification.