tarantool_instance doesn't do anything if tarantool_box isn't found

A fix for https://bugs.launchpad.net/tarantool/+bug/992171
"SAVE SNAPSHOT process (dumper) does not respond to signals"

Reset the signal mask/handler after a fork to their defaults.
Make sure that when the snapshot dumper process is terminated
with a signal, we report it back to the user.

No test case since it would be hard to do a concurrent
test in our framework yet.

When salloc_init() fails, we call tarantool_free(),
which calls fiber_free(), which tries to destroy the fiber registry
hash, which has not been allocated.

Check that the hash has been allocated, before trying to destroy it.

Fix bug https://bugs.launchpad.net/tarantool/+bug/990728
"box.space:truncate() is a CPU hog".

The old truncate algorithm would restart iteration
from the beginning to delete every tuple.
With HASH index it was especially slow, since
it would have to skip more and more empty slots
to find a non-empty slot for deletion.

Rewrite the truncate() to walk linearly
over all elements and delete them.

This works both with HASH and TREE indexes.

On the same token make truncate multipart-index
aware (was missed in multipart-pk patch).
Add a test for that.

To work with multipart keys efficiently,
implement tuple:slice(). A slice returns
a linear set of fields from a tuple.
Convenient to retreive a multipart key or
tuple tail. Document tuple:slice().

Convert box.lua to box.lua.c and compile it in.
Fix broken out-of-source build of luajit, by adding
a few more generated files to touch-nocreate rule.