Rearrange internal header files.

This commit rearranges the internal header files.

"src/guestfs-internal.h" is just for the library, as before.

"src/guestfs-internal-frontend.h" is for use by all library, bindings,
tools C code, but NOT the daemon.

"src/guestfs-internal-all.h" is for use by all C code including the
daemon.

This is just code motion, but it has some important consequences:

(1) We can use the CLEANUP_* macros in bindings and tools code.

(2) We can get rid of TMP_TEMPLATE_ON_STACK.

(3) We will (in future) be able to stop bindings and tools code from
using the safe_* allocation functions (which are NOT safe to use
outside the library alone).
Remove all occurrences of the bad_cast (lowercase) function.

Not to be confused with the libxml2 macro 'BAD_CAST' which converts
from 'signed char *' to 'unsigned char *'.

The 'bad_cast' function was defined and used all over the place as a
replacement for a '(char *)' cast.  I think it is better to make these
casts explicit, instead of hiding them in an obscure function.
Remove TMP_TEMPLATE_ON_STACK macro.