Add maint set/show max-worker-threads

Add maint set/show max-worker-threads

This adds maint commands to control the maximum number of worker
threads that gdb can use.

2019-09-30  Tom Tromey  <tom@tromey.com>

	* NEWS: Add entry.
	* maint.c (_initialize_maint_cmds): Add "max-worker-threads" maint
	commands.

gdb/doc/ChangeLog
2019-09-30  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Maintenance Commands): Document new maint
	commands.