watch: wire up IMAP IDLE reapers to DS

watch: wire up IMAP IDLE reapers to DS

We can avoid synchronous `waitpid(-1, 0)' and save a process
when simultaneously watching Maildirs.

One DS bug is fixed: ->Reset needs to clear the DS $in_loop flag
in forked children so dwaitpid() fails and allows git processes
to be reaped synchronously.  TestCommon also calls DS->Reset
when spawning new processes, since t/imapd.t uses DS->EventLoop
while waiting on -watch to write.