add system for resetting TLS to initial values

this is needed for reused threads in the SIGEV_THREAD timer
notification system, and could be reused elsewhere in the future if
needed, though it should be refactored for such use.

for static linking, __init_tls.c is simply modified to export the TLS
info in a structure with external linkage, rather than using statics.
this perhaps makes the code more clear, since the statics were poorly
named for statics. the new __reset_tls.c is only linked if it is used.

for dynamic linking, the code is in dynlink.c. sharing code with
__copy_tls is not practical since __reset_tls must also re-zero
thread-local bss.

src/time/timer_create.c

@@ -19,6 +19,8 @@ static void dummy_1(pthread_t self)
 }
 weak_alias(dummy_1, __pthread_tsd_run_dtors);
 
+void __reset_tls();
+
 static void cleanup_fromsig(void *p)
 {
 	pthread_t self = __pthread_self();
@@ -28,6 +30,7 @@ static void cleanup_fromsig(void *p)
 	self->canceldisable = 0;
 	self->cancelasync = 0;
 	self->unblock_cancel = 0;
+	__reset_tls();
 	longjmp(p, 1);
 }