Fix bug 1263087 (Percona Server 5.5 / 5.6 do not handle partial /
interrupted AIO calls) by backporting the os_aio_linux_handle() bits
of lp:mysql-server revision 4334.

The backport is straightforward with mechanical os_mutex/mutex_t
bool/ibool changes.
Automerge lp:~laurynas-biveinis/percona-server/bug1263087-5.5
Fix for bug #1266980 crash in group_concat ported from MySQL 5.5.
Test case.
Add testcase for bug #1266980.
Automerge lp:~sergei.glushchenko/percona-server/5.5-ps-bug1266980