Update prof defaults to match common usage.

Change the "opt.lg_prof_sample" default from 0 to 19 (1 B to 512 KiB).

Change the "opt.prof_accum" default from true to false.

Add the "opt.prof_final" mallctl, so that "opt.prof_prefix" need not be
abused to disable final profile dumping.

src/jemalloc.c

@@ -561,6 +561,7 @@ malloc_conf_init(void)
 				    lg_prof_interval, -1,
 				    (sizeof(uint64_t) << 3) - 1)
 				CONF_HANDLE_BOOL(opt_prof_gdump, prof_gdump)
+				CONF_HANDLE_BOOL(opt_prof_final, prof_final)
 				CONF_HANDLE_BOOL(opt_prof_leak, prof_leak)
 			}
 			malloc_conf_error("Invalid conf pair", k, klen, v,