The problem was that the multiple evaluations of a ENCODE or
DECODE function within a single statement caused the random
generator to be reinitialized at each evaluation, even though
the parameters were constants.

The solution is to initialize the random generator only once
if the password (seed) parameter is constant.

This patch borrows code and ideas from Georgi Kodinov's patch.

mysql-test/r/func_str.result:
  Add test case result.
mysql-test/r/ps.result:
  Add test case result.
mysql-test/t/func_str.test:
  Add test case for Bug#49141
mysql-test/t/ps.test:
  Add test case for Bug#49141
sql/item_strfunc.cc:
  Move seed generation code to a separate method.
  Seed only once if the password (seed) argument
  is constant.
  Remove duplicated code and use a transform method
  to apply encoding or decoding.
sql/item_strfunc.h:
  Add parameter to signal whether the PRNG is already seeded.
  Introduce transform method.
  Combine val_str methods.
sql/sql_crypt.cc:
  Remove method.
sql/sql_crypt.h:
  Seed is supplied as two long integers.