turned ProtobufCFieldDescriptor->packed into ->flags
minor adjustments to the field flags patch

rename PROTOBUF_C_FIELD_FLAGS_PACKED to PROTOBUF_C_FIELD_FLAG_PACKED.

rename ProtobufCFieldFlagType to ProtobufCFieldFlag.

wrap some particular long lines.

update documentation.

for clarity, use a "uint32_t" instead of "unsigned" for the 'flags'
field in _ProtobufCFieldDescriptor.
added deprecated flag into ProtobufCFieldDescriptor->flags

[edmonds@fsi.io: resolve merge conflicts, add documentation]
t/: add test case test_field_flags
ChangeLog: credit Andrei Nigmatulin for #138