There was auto-reconnecting by slave earlier than a prescribed by slave_net_timeout value.
The issue happened on 64bit solaris that spotted rather incorrect casting of 
the ulong slave_net_timeout into the uint of mysql.options.read_timeout.

Notice, that there is no reason for slave_net_timeout to be of type of ulong.
Since it's primarily passed as arg to mysql_options the type can be made
as uint to avoid all conversion hassles.
That's what the fixes are made.
A "side" effect of the patch is a new value for the max of slave_net_timeout
to be the max of the unsigned int type (therefore to vary across platforms).

Note, a regression test can't be made to run reliably without making it to last over some 
20 secs. That's why it is placed in suite/large_tests.

mysql-test/suite/large_tests/r/rpl_slave_net_timeout.result:
  the new test results.
mysql-test/suite/large_tests/t/rpl_slave_net_timeout-slave.opt:
  Initialization of the option that yields slave_net_timeout's default.
sql/mysql_priv.h:
  changing type for slave_net_timeout from ulong to uint
sql/mysqld.cc:
  changing type for slave_net_timeout from ulong to uint
sql/sys_vars.cc:
  Refining the max value for slave_net_timeout to be as the max for uint type.