pt-mysql-summary checks installed mysqld binary which may not be the version in memory

Any time 'which mysqld' is used, if the $pid is known, we should instead use /proc/${pid}/exe as the version returned from which could have changed during updates.

For example in the following line:

grep -qc jemalloc /proc/${pid}/environ || ldd $(which mysqld) 2>/dev/null | grep -qc jemalloc