init: Avoid outputting non-ASCII characters from -init_debug

ba18068dc28693 (#6685) added printing of timing for the `-init_debug`
option:

    {start,heart}
    {'done_in_µs',10}

With that change, the test case `peer_SUITE:init_debug/1` started to
fail sporadically. That test case tries to start a peer node with
option `-init_debug` using the connection type `standard_io`:

    {ok, Peer, _Node} = peer:start_link(#{name => peer:random_name(?FUNCTION_NAME),
        shutdown => 1000,
        connection => standard_io, args => ["-init_debug"]}),

The test case would fail when the two bytes that comprise the `µ`
character were split up into two separate IO requests.

We could update the `peer` module so that it would do buffering and
re-combine split UTF-8 sequences. That would fix the failing test
case. However, introducing re-combining is not sufficient to make it
safe to print arbitrary non-ASCII characters, because of an
undocumented restriction mentioned in a comment for the `standard_io`
connection type:

    %% Characters in range of 192-255 are reserved for control sequences,
    %%  see encode_port_data for details. If peer node attempts to print
    %%  characters in this range, an controlling process on the origin
    %%  node may terminate with an error (because CRC check will fail).

(The actual restriction is actually slightly less severe than the
comment implies, which is why `µ` would work after re-combining but
others characters such as `ö` would not. Note that the other connection
modes don't have this restriction, but they are not able to catch the
output from `-init_debug`.)

Since printing of arbitrary non-ASCII characters cannot be made safe
for the `standard_io` connection mode, change the printout of timing
information to only use ASCII characters:

    {start,heart}
    {done_in_microseconds,10}

2 files changed, 1 insertions, 1 deletions

diff --git a/erts/preloaded/ebin/init.beam b/erts/preloaded/ebin/init.beam
index 0e7296924f..d2ea6b7025 100644
--- a/erts/preloaded/ebin/init.beam
+++ b/erts/preloaded/ebin/init.beam
diff --git a/erts/preloaded/src/init.erl b/erts/preloaded/src/init.erl
index 45ca42778d..7933cfaa34 100644
--- a/erts/preloaded/src/init.erl
+++ b/erts/preloaded/src/init.erl
@@ -106,7 +106,7 @@ debug(_, T, Fun) ->
     Val = Fun(),
     T2 = erlang:monotonic_time(),
     Time = erlang:convert_time_unit(T2 - T1, native, microsecond),
-    erlang:display({'done_in_μs', Time}),
+    erlang:display({done_in_microseconds, Time}),
     Val.
 
 -spec get_configfd(integer()) -> none | term().