* john/tools/fix-cover-register-race/ERL-943/OTP-15813:
  cover: Fix register/2 race on startup

Use `counters:add/3` instead of `ets:update_counter/3`
for counting the number of times a line is executed.

By default, the reference to the counter array for each module will be
stored in a persistent term and retrieved every time a counter is
updated. This makes the compiler test suite with coverage enabled run
*almost* twice as fast (on my computer, in about 6 minutes down from
more than 11 minutes).

To get even more speed, the new `cover:local_only/0` function can be
called to put cover into a mode where the cover-compiled code can only
be run on the local node. In this mode, the cover-compiled modules in
a more efficient way by compiling the counter reference into the
code. This shaves off about one more minute, making the compiler test
suite with coverage enabled run *more than* twice as fast (in about 5
minutes on my computer).

Prior to PR#1807, uncovered lines could be found by searching for "
0.." in the HTML page generated by cover:analyse_to_file/1,2. The pull
request removed the two dots after the number of hits, which makes it
much harder to find the uncovered lines.

This commit introduces a sad face, :-( , instead of the single 0
character, which should make the search easier.

This will make it easier to send links to other people.

add missing tags too