The SET PASSWORD statement is non-transactional (no explicit transaction
boundaries) in nature and hence is forbidden inside stored functions and
triggers, but it weren't being effectively forbidden.

The implemented fix is to issue a implicit commit with every SET PASSWORD
statement, effectively prohibiting these statements in stored functions
and triggers. 


mysql-test/r/sp-error.result:
  Add test case result for Bug#30904
mysql-test/t/sp-error.test:
  Add test case for Bug#30904
sql/sql_lex.h:
  Add variable to set that a statement with SET PASSWORD causes a implicit
  commit.
sql/sql_parse.cc:
  End active transaction in SET PASSWORD.
sql/sql_yacc.yy:
  Set the correct flag on SET PASSWORD if inside a SP, thus effectively
  prohibiting SET PASSWORD statements in stored functions and triggers.