mysqld daemon should check if any process uses the socket file before removing

Already reported to http://bugs.mysql.com/71194

When running some MySQL daemon A and we are trying to run another instance B, while these conditions are met:

• network ports are different for A and B
• unix socket location is the same for A and B

Then the new daemon B removes the unix socket file that is actually necessary for daemon A.

How to repeat:
Steps to reproduce:
$ /usr/libexec/mysqld --port 13306 --datadir /var/lib/mysql/
$ fuser /var/lib/mysql/mysql.sock
$ /usr/libexec/mysqld --port 13307 --datadir /var/lib/mysql2/
$ fuser /var/lib/mysql/mysql.sock

Actual results:
/var/lib/mysql/mysql.sock: 5683
/var/lib/mysql/mysql.sock: 5717
which means the first daemon is not able to accept connections on the unix socket

Expected results:
/var/lib/mysql/mysql.sock: 5683
/var/lib/mysql/mysql.sock: 5683
the second daemon shouldn't start at all

Suggested fix:
Either check if some proc is attached to the socket or (portable solution) having a lock file for the socket file, that would contain pid of the process using the socket file.