Compute the size of the InnoDB buffer pool based on memory

Add a variable INNODB_BUFFER_POOL_SIZE_PCT that indicates how much of
the main memory should be alllocated to the InnoDB buffer pool and use
it to compute INNODB_BUFFER_POOL_SIZE. The overwrite via the environment
variables remains available. The default value for the new variable is
80. Also fix the indentation for run-mysqld.