Constify thread name return path

Constify thread name return path

Since this code path returns a string owned by the target (we don't know how
it's allocated, could be a static read-only string), it's safer if we return
a constant string.  If, for some reasons, the caller wishes to modify the
string, it should make itself a copy.

gdb/ChangeLog:

	* linux-nat.c (linux_nat_thread_name): Constify return value.
	* target.h (struct target_ops) <to_thread_name>: Likewise.
	(target_thread_name): Likewise.
	* target.c (target_thread_name): Likewise.
	* target-delegates.c (debug_thread_name): Regenerate.
	* python/py-infthread.c (thpy_get_name): Constify local variables.
	* thread.c (print_thread_info): Likewise.
	(thread_find_command): Likewise.