Removed hard-coded error messages. All messages are now in
errmsg.txt
Also renamed enumeration value BINLOG_STMT_UNSAFE_FUNCTION to
BINLOG_STMT_UNSAFE_SYSTEM_FUNCTION to make the naming consistent
with BINLOG_STMT_UNSAFE_SYSTEM_VARIABLE.


sql/item_create.cc:
  Renamed BINLOG_STMT_UNSAFE_FUNCTION to
  BINLOG_STMT_UNSAFE_SYSTEM_FUNCTION
sql/share/errmsg.txt:
  Moved hard-coded strings from THD::issue_unsafe_warnings() to
  errmsg.txt.
sql/sql_class.cc:
  - Moved error messages to errmsg.txt.
  - Updated comment above THD::issue_unsafe_warnings().
sql/sql_lex.h:
  Renamed BINLOG_STMT_UNSAFE_FUNCTION to
  BINLOG_STMT_UNSAFE_SYSTEM_FUNCTION
sql/sql_yacc.yy:
  Renamed BINLOG_STMT_UNSAFE_FUNCTION to
  BINLOG_STMT_UNSAFE_SYSTEM_FUNCTION