Extract the non-failing parts of the original patch by Roman.
Make an attempt at finding why coeio_custom is crashing when
replication_source is changed quickly - no success yet.

Duplicate log_fd to STDOUT_FILENO on say_logrotate()

See also 0a1780cb

Tarantool's signal handlers are now installed *before* executing
Lua script. libreadline doesn't install own signals anymore.
TTY is now clean up properly in all cases.

This patch also fixes possible problems with signal handlers
after daemonization via box.cfg { background = true }.

Fix some bugs in the function (now it uses libc's (3) dirname ffi.call).

A dropped user structure could still be accessed from sessions
which were authenticated with this user.

Check that a user is dropped when accessing it from the authenticated
session.

 + Add tarantoolctl enter instance_name command

This patch also changes box.info.server output for this case.

Avoid uncontrolled growth of region memory.

Stop producing "excessively sparse array"

Other changes:

 - "" or NULL=ffi.cast('void *', 0) now resets option to default value
 - app/cfg.test.lua converted to TAP format