Dumb programs which have a --daemon option call fork() early. This is
terrible UX since startup errors get lost: the program exits with
"success" immediately then you discover via the logs that it didn't
start at all.
However, forking late introduced a heap of problems with changing
pids. Instead, fork early but keep stderr and the parent around: if
we fail early on, the parent fails with us. We release our parent
with an explicit action just before the main loop.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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