info: add Name to MapInfo

Allow users to access the map name. The name is empty if the kernel
doesn't support object names yet.

btf: avoid Type copy in FuncProto.walk

FuncProto.walk currently takes a copy of FuncParam.Type since the
for loop implicitly makes a copy of the FuncParam value:

    for _, m := range fp.Params {

m is a copy of the original param, so &m.Type is the address of a
temporary variable. Instead, iterate params using the index and
take the correct address. Also extend the tests to cover this case.

map: add a getter for Flags

Add a getter for MapABI.Flags, so that we can deprecate MapABI without
losing access to Flags.