- Create space safe strings for system() calls in mysql_upgrade.exe


client/mysql_upgrade.c:
  Bug#22807 mysql_upgrade fails when called with a basedir-path containing spaces
  - Make use of new dynstr_append_os_quoted function to produce a string safe for
  passing to the system() function.                                              
  - Refactor possible source of assertion.
include/my_sys.h:
  Bug#22807 mysql_upgrade fails when called with a basedir-path containing spaces
  - Declare dynstr_append_os_quoted function.
mysys/string.c:
  Bug#22807 mysql_upgrade fails when called with a basedir-path containing spaces
  - Define dynstr_append_os_quoted function.  This function will concatenate any 
  number of strings, escapes any OS quote in the result then surround the whole  
  affair in another set of quotes which is finally appended to specified         
  DYNAMIC_STRING.