timeout: fix failure if timeout's parent has ignored SIGCHLD

timeout: fix failure if timeout's parent has ignored SIGCHLD

* src/timeout.c (main): Reset the SIGCHLD handler to the default
as otherwise wait() could return -1 and set errno to ECHILD.
This condition was ignored until commit 0b1dcf33, on 31-08-2009,
"timeout: defensive handling of all wait() errors"
but subsequently timeout would run the command correctly
but then fail with an error message.
* tests/misc/timeout: In a subshell set the CHLD handler to
SIG_IGN to ensure the timeout command resets it to SIG_DFL.
* NEWS: Mention the fix.