pipe: add a new macro FLB_PIPE_WOULDBLOCK
pipe: add a new macro FLB_PIPE_WOULDBLOCK

This macro deprecates flb_pipe_check_eagain().

This uses the design we employed to implement FLB_WOULDBLOCK()
in flb_socket.h, which seems the best one for this task.

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@clear-code.com>

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