Extract bsdsocket_io_wait() from bsdsocket.cc to public C API.
This very simple patch allow to use Tarantool event machine in
stored C procedures.

Other changes: replace fiber_wakeup() with fiber_call() here to save extra
               event loop iteration.

A prerequisite for #1062: Automatic cluster bootstrap

Updated libeio invokes done_poll callback but we weren't providing one.

Wrong fix.

This reverts commit cd200c65.

Remove broadcast(): it din't work as expected and wasn't used
by anyone.

Enable monotonic timers (if available).

Enable floor() usage in computations (instead of a crude approximation
with bithacks).

Enable eventfd and signalfd usage on Linux.

Update error messages.
Follow the coding style in C function naming.

Added lots of checks and nice error messages.
Added lots of tests

Disallow upsert in presence of a unique secondary key.

Update tests.
Fix typos.

_Noreturn kw must appear before the function name as per C11 grammar and
clang7 is the first one to enforce that.

Ecb_noreturn macro is defined using _Noreturn keyword if C11 support is
detected; otherwise __noreturn__ gcc attribute is used. The later must
appear after the function parameters list.  These two methods to
annotate a function are inherently incompatible in terms of the token
location relative to the function declaration.

Use gcc attribute exclusively since it is always available.

Conflicts:
	src/box/relay.cc