Use SCM_P instead of PROTO; the latter intrudes on the user's

Use SCM_P instead of PROTO; the latter intrudes on the user's
namespace.
* params.h: Deleted; definition of SCM_P moved to...
* __scm.h: ... here, where it replaces PROTO macro.
* libguile.h, smob.h: Don't #include "params.h".
* continuations.c, error.h, feature.h, gc.c, gc.h, init.h, load.h,
smob.h: Fix prototypes accordingly.
* Makefile.in: Update dependencies.
(inner_h_files): Remove params.h.