Merge lp://qastaging/~tplavcic/percona-server/bug1382069-5.6 into lp://qastaging/percona-server/5.6

Mysqld_safe was loading system jemalloc if installed no matter if user specified another malloc lib - so it was possible that it tries to load 2 libs. Also there was no possibility to disable loading of system jemalloc.
With this change system jemalloc is only loaded if no other lib is specified and if user specifies "malloc-lib=" nothing will be loaded.

Param job is here:
http://jenkins.percona.com/view/PS%205.6/job/percona-server-5.6-param/742/

Some testing:
TEST1: nothing in my.cnf (automatically loads system jemalloc)
======
vagrant@t-ubuntu14:~$ sudo /usr/bin/mysqld_safe
141021 00:47:57 mysqld_safe Adding '/usr/lib/x86_64-linux-gnu/libjemalloc.so.1' to LD_PRELOAD for mysqld
141021 00:47:57 mysqld_safe Logging to '/var/log/mysql/error.log'.
141021 00:47:57 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

TEST2: malloc-lib= (loads nothing)
======
vagrant@t-ubuntu14:~$ sudo /usr/bin/mysqld_safe
141021 00:54:35 mysqld_safe Logging to '/var/log/mysql/error.log'.
141021 00:54:35 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

TEST3: malloc-lib=/home/vagrant/ps/lib/mysql/libjemalloc.so.1 (loads only specified lib)
======
vagrant@t-ubuntu14:~$ sudo /usr/bin/mysqld_safe
141021 00:55:57 mysqld_safe Adding '/home/vagrant/ps/lib/mysql/libjemalloc.so.1' to LD_PRELOAD for mysqld
141021 00:55:57 mysqld_safe Logging to '/var/log/mysql/error.log'.
141021 00:55:57 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql