Bug#34678 @@debug variable's incremental mode

The problem is that the per-thread debugging settings stack wasn't
being deallocated before the thread termination, leaking the stack
memory. The chosen solution is to push a new state if the current
is set to the initial settings and pop it (free) once the thread
finishes.


dbug/dbug.c:
  Move dbug parser out of _db_set_ to a separate function and
  make _db_set_ push a new stack if the corrent one is set to
  the initial settings.
dbug/user.r:
  Update DBUG_SET description.
mysql-test/t/disabled.def:
  Re-enable test case which triggered the leak.
mysys/my_thr_init.c:
  Pop a pushed state, nop if stack is empty.
sql/set_var.cc:
  Handle incremental debug settings.
mysql-test/r/variables_debug.result:
  Add new test case result for Bug#34678
mysql-test/t/variables_debug.test:
  Add new test case for Bug#34678