Rewrite Rhizome manifest parsing

Move validation checks into new function rhizome_manifest_validate()

Remove rhizome_manifest 'errors' field

Replace rhizome_manifest 'warnings' with 'malformed'

Replace rhizome_manifest 'manifest_bytes' with 'manifest_body_bytes' and
refactor to use 'manifest_all_bytes' in all manifest i/o

Refactor rhizome_manifest_verify() and reverse sense of return value to
match rhizome_manifest_validate()

New function rhizome_manifest_inspect() -- lightweight manifest parser
used when receiving Rhizome advertisements

New 'rhizomeops' test case for invalid manifest "service" field values,
now passes
Minor refactor in strn_edup()
Better error logging for recvwithttl()