libknots: public interface cleanup

Currently src/Makefile.am contains following ugliness:

libknot_la_LDFLAGS = ... -export-symbols-regex '^(knot|KNOT|rrset|tsig|zone)_'

Following steps should be taken to resolve this issue:

1. Cleanup all symbols in the library to start with knot_ or KNOT_ prefix.
2. Macros defining the symbol visibility should be used to decorate the functions in headers [1]:
3. The symbol regex should be dropped and gl_VISIBILITY() re-enabled in configure.ac

[1]:

#define KNOT_PUBLIC  __attribute__ ((visibility ("default")))
#define KNOT_LOCAL  __attribute__ ((visibility ("hidden")))

@ondrej