Turn off threaded minsym demangling by default

Turn off threaded minsym demangling by default

Per discussion on gdb-patches with Joel, this patch turns off multihreaded
symbol loading by default. It can be turned on using:
  maint set worker-threads unlimited

To keep the behavior as close as possible to the old code, it still
calls symbol_set_names in the old place if n_worker_threads is 0.

gdb/ChangeLog:

2019-11-27  Christian Biesinger  <cbiesinger@google.com>

	* maint.c (n_worker_threads): Default to 0.
	(worker_threads_disabled): New function.
	* maint.h (worker_threads_disabled): New function.
	* minsyms.c (minimal_symbol_reader::record_full): Call symbol_set_names
	here if worker_threads_disabled () is true.
	(minimal_symbol_reader::install): Skip all threading if
	worker_threads_disabled () is true.

Change-Id: I92ba4f6bbf07363189666327cad452d6b9c8e01d