The code to get read the value of a system variable was extracting its value 
on PREPARE stage and was substituting the value (as a constant) into the parse tree.
Note that this must be a reversible transformation, i.e. it must be reversed before
each re-execution.
Unfortunately this cannot be reliably done using the current code, because there are
other non-reversible source tree transformations that can interfere with this
reversible transformation.
Fixed by not resolving the value at PREPARE, but at EXECUTE (as the rest of the 
functions operate). Added a cache of the value (so that it's constant throughout
the execution of the query). Note that the cache also caches NULL values.
Updated an obsolete related test suite (variables-big) and the code to test the 
result type of system variables (as per bug 74).

mysql-test/extra/rpl_tests/rpl_insert_id.test:
  Bug #32124: removed ambiguous testcase
mysql-test/r/innodb_data_home_dir_basic.result:
  Bug #32124: fixed wrong test case
mysql-test/r/innodb_flush_method_basic.result:
  Bug #32124: fixed wrong test case
mysql-test/r/ps_11bugs.result:
  Bug #32124: test case
mysql-test/r/ssl_capath_basic.result:
  Bug #32124: fixed wrong test case
mysql-test/r/ssl_cipher_basic.result:
  Bug #32124: fixed wrong test case
mysql-test/r/variables.result:
  Bug #32124: system vars are shown as such in EXPLAIN EXTENDED, not as constants.
mysql-test/suite/rpl/r/rpl_insert_id.result:
  Bug #32124: removed ambiguous testcase
mysql-test/t/ps_11bugs.test:
  Bug #32124: test case
sql/item.cc:
  Bug #32124: placed the code to convert string to longlong or double 
  to a function (so that it can be reused)
sql/item.h:
  Bug #32124: placed the code to convert string to longlong or double 
  to a function (so that it can be reused)
sql/item_func.cc:
  Bug #32124: moved the evaluation of system variables at runtime (val_xxx).
sql/item_func.h:
  Bug #32124: moved the evaluation of system variables at runtime (val_xxx).
sql/set_var.cc:
  Bug #32124: removed the code that calculated the system variable's value 
  at PREPARE
sql/set_var.h:
  Bug #32124: removed the code that calculated the system variable's value 
  at PREPARE
tests/mysql_client_test.c:
  Bug #32124 : removed the reading of the system variable, because its max
  length is depended on the system charset and client charset and can't be
  easily calculated.