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.
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