lib/fuzzing: Split up automatically build fuzzers into TYPE_{IN,OUT,STRUCT}

lib/fuzzing: Split up automatically build fuzzers into TYPE_{IN,OUT,STRUCT}

The advise is that a fuzz target should be as small as possible
so we split this up.  Splitting up by function would build too
many fuzzers, but this should help a little.

See for example:
https://github.com/google/fuzzing/blob/master/docs/good-fuzz-target.md#large-apis

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@samba.org>