tail: fix race condition (#3798)

* tail: fix race condition (fix #3765)

There exists a race condition (RC) that can occur if changes to a path
happen after the initial print loop in `uu_tail()`, but before the
path is added to the notify-Watcher thread in `follow()`.

To minimize the window where the RC can occur, this moves starting the
Watcher thread and adding paths to it from `follow()` to the initial
print loop in `uu_tail()`.

Additionally, to make sure the RC cannot happen in
"gnu/tests/tail-2/F-headers.sh", the error message that is used as a trigger
in this test, is delayed until the path is added to the Watcher thread.

* build-gnu: remove workarounds for tail

Remove workarounds for "tests/tail-2/F-headers.sh" which are
(presumably) no longer needed because of the race condition fix.

* build-gnu: remove workarounds for tail

Remove workarounds for "tests/tail-2/F-headers.sh" which are
(presumably) no longer needed because of the race condition fix.

* tail: refactor to minimize chances of RC

Move "adding paths to Watcher thread" to its own loop and run this loop
before the initial tail-print-loop in order to minimize the window for
race conditions.

util/build-gnu.sh

@@ -168,10 +168,6 @@ sed -i -e "s|removed directory 'a/'|removed directory 'a'|g" tests/rm/v-slash.sh
 # however there's a bug because `---dis` is an alias for: `---disable-inotify`
 sed -i -e "s|---dis ||g" tests/tail-2/overlay-headers.sh
 
-# F-headers.sh test sometime fails (but only in CI),
-# just testing inotify should make it more stable
-sed -i -e "s|echo x > a|sleep .1; echo x > a; sleep .1|g" -e "s|echo y > b|sleep .1; echo y > b; sleep .1|g" -e "s| '---disable-inotify'||g" tests/tail-2/F-headers.sh
-
 test -f "${UU_BUILD_DIR}/getlimits" || cp src/getlimits "${UU_BUILD_DIR}"
 
 # When decoding an invalid base32/64 string, gnu writes everything it was able to decode until