Use the magic snarfer.

Use the Guile magic snarfer to make guile_ssh_* functions visible to
the Scheme world.  Use functions names as they seen from Scheme
instead of C names in type check macros.

* src/auth.c, src/channel-func.c, src/channel-type.c,
  src/key-func.c, src/key-type.c, src/session-func.c,
  src/session-type.c, src/version.c: Use snarfing macros.
* src/Makefile.am: Use guile-snarf to generate needed files.  Fix
  some comments.
* configure.ac: Use AM_SILENT_RULES.
* .gitignore: Ignore files produced by the magic snarfer.