A 'tentative definition' in C language is a global variable definition
missing both a storage class and an initializer, ex: int foo.
Traditionally it get's translated into a common symbol, allowing for
multiple definitions in different translation units to coexist.

Normally multiple definitions aren't allowed; unless they are common
symbols.  In the later case a linker picks one arbitrarily.

-fno-common alters tentative definitions translation, resulting in
a more strict and conformant semantics, multiple definitions are no
longer allowed.

This change was motivated by the OSX linker not handling common symbols
in static libraries
(https://stackoverflow.com/questions/19398742/os-x-linker-unable-to-find-symbols-from-a-c-file-which-only-contains-variables)

Avoid throwing exceptions from most fiber functions.

say.cc -> say.c

object.h was an Objective C legacy providing nothing, mostly, and with some
changes to class hierarchies I have in mind using it will introduce multiple
inheritance, which I'd rather avoid.

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.

Support the following configuration:

    box.cfg {
        replication_source = {
            "uri1";
            "uri2";
            "uri3";
            -- ...
        }
    }

This change is backward-compatible: old syntax is supported as well.
box.info.replication is not changed and displays state of the first
(if sort by URI) master.

This patch also adds a simple hack to automatically detect the first
master (if sorted by URI) during initial cluster bootstrap by
comparing box.cfg.listen uri with replication_source uris.

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.