PAM upports binary conversations using private protocols, this
can be handled by C but it's not supported here because we
implicitly convert all the messages to string, and this may lead
to issues when this is not the case (as in binary protocol the
pointer could contain zeros that the GoString conversion would
consider them the end of the message).
So, add another conversation handler implementation that allows
to handle the binary protocol, whose function callback accepts
a pointer to the struct (we can't use bytes as the length is
unknown and may be defined in the header of the pointer itself).
However since the binary prompt is not supported by all the
platforms we need to do a compile-time check to disable it in
case is used when not supported.
Go provides a nicer way to handle Go structs lifetime when they
are passed to C now, so use this instead of a custom
implementation that requires to store them in a map
PAM has a pam_start_confdir() which allows to define the configuration
directory where all services are located.
This is useful to define your own service on tests in particular, so
that you can control your stack and be independant of the host when
running them.
Allow defining this configuration directory, with a new StartConfDir
function.
Also, allow pre-checking for the API availability with
CheckPamHasStartConfdir().
