core/service: use array rather than list for extra fds, limit max number

core/service: use array rather than list for extra fds, limit max number

Follow-up for 3543456f84ec2e83e07b6c9bf2b3a1c5d30241d8

I don't think list is particularly useful here. The passed fds are
constant for the lifetime of service, and with this commit we track
the number of extra fds in a dedicated var anyway.