Add define YASSL_PREFIX when compiling yassl

Import patch from yaSSL
- avoid allocating memory for each call to 'EVP_md5' and 
  'EVP_des_ede3_cbc' which were not released until server was stopped
- Those functions are used from the SQL function 'des_encrypt' and
  'des_decrypt'.

Add new define YASSL_PREFIX beforee including ssl.h to activate inclusion of prefix_*.h files

Bug#20022 mysql-test-run can't be run with secure connections turned on for all testcases
- Part 1, fixes rpl- and federated-tests where connection is made to 127.0.0.1

- Include prefix files that renames all public functions in yaSSLs
  OpenSSL API to ya<function_name>. They will otherwise conflict
  with OpenSSL functions if loaded by an application that uses OpenSSL
  as well as libmysqlclient with yaSSL support.

Bug#18235: assertion/crash when windows mysqld is ended with ctrl-c
  
Two threads both try a shutdown sequence which creates a race to the
de-init/free of certain resources.
  
This exists in similar form in the client as 17926: "mysql.exe crashes
when ctrl-c is pressed in windows."

Update after merge to 5.0

BUG#18669: Session COM_STATISTICS breaks mysqladmin status.
Changed COM_STATISTICS to display the global status, instead of thead status, for slow queries and table opens.

- In function 'handle_grant_struct' when searching the memory structures for an 
  entry to modify, convert all entries here host.hostname is NULL to "" and compare that 
  with the host passed in argument "user_from".
- A user created with hostname "" is stored in "mysql.user" table as host="" but when loaded into 
  memory it'll be stored as host.hostname NULL. Specifiying "" as hostname means
  that "any host" can connect. Thus is's correct to turn on allow_all_hosts
  when such a user is found. 
- Review and fix other places where host.hostname may be NULL.

BUG#19394 OPT_INNODB_THREAD_CONCURRENCY duplicated
Removed duplication (not a user-visible change)