myisam_repair_threads > 1 causes crash

Problem: parallel repair (myisam_repair_threads > 1) of a myisam
table with two or more fulltext keys that use the same parser may
lead to a server crash. ALTER TABLE ENABLE KEYS is affected as well.

Fix: properly initialize fulltext structures for parallel repair.

Note: 1. there's no deterministic test case.
2. now we call parser->init() for each fulltext key
(not for each fulltext parser used).


storage/myisam/ft_parser.c:
  Fix for bug #37756: enabling fulltext indexes with
    myisam_repair_threads > 1 causes crash
  
  In ftparser_call_initializer() we "group" parsers
  and allocate parameters for each unique parser used.
  In case of parallel repairing we may have a bunch
  of parsers that use the only MI_INFO structure.
  Each of these parsers must have its own parameter
  structure in order not to interfere with others.
  
  Moreover, the allocation is done without mutex
  lock so parallel ftparser_call_initializer() calls
  are unsafe.
  
  Now we don't "group" the fulltext parsers.
  Parameter allocation code moved to ftparser_alloc_param()
  function which is called from mi_repair_parallel() as well
  to allocate parameters for each parser used.
storage/myisam/ftdefs.h:
  Fix for bug #37756: enabling fulltext indexes with
    myisam_repair_threads > 1 causes crash
  
  ftparser_alloc_param(MI_INFO *info) added.
storage/myisam/mi_check.c:
  Fix for bug #37756: enabling fulltext indexes with
    myisam_repair_threads > 1 causes crash
  
  ftparser_alloc_param(info) called in case of parallel
  repair to allocate fulltext parser parameters.
storage/myisam/mi_open.c:
  Fix for bug #37756: enabling fulltext indexes with
    myisam_repair_threads > 1 causes crash
  
  set keyinfo->ftkey_nr and share->ftkeys.