Allow private functions to be included in clib/proto file

[tboeckel]

Currently sfdc will omit all private functions when generating the clib/proto file.

For example having function definitions like this:

==public
void public_foobar(int param) (d0)
==private
void private_foobar(int param) (d0)

Then call sfdc like this:
sfdc --mode=clib foobar_lib.sfd

The generated file will contain a prototype for public_foobar(), but not for private_foobar(). This might be desired when generating proto files for the public, but for generating private proto files it is not very convenient. The same applies for --mode=macros.

However, with --mode=gateproto, --mode=gatestubs, --mode=interface (and possibly some more modes) even private functions are included in the output.