trailers: export action enums and corresponding lookup functions

trailers: export action enums and corresponding lookup functions

Separate the mechanical changes out of the next patch.  The functions
are changed to take a pointer to enum, because struct conf_info is not
going to be public.

Set the default values explicitly in default_conf_info, since they are
not anymore close to default_conf_info and it's not obvious which
constant has value 0.  With the next patches, in fact, the values will
not be zero anymore!

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>