shrink pipes under Linux

We have never had any need for pipes with the default 64K
capacity on Linux.  Our pipes are only used for tiny writes
in signal handlers and to perform parent shutdown detection.

With the current /proc/sys/fs/pipe-user-pages-soft
default, only 1024 pipes can be created by an unprivileged
user before Linux clamps down the pipe size to 4K (a single page)
for newly-created pipes[1].

So avoid penalizing OTHER pipe users who could benefit from the
increased capacity and use only a single page for ourselves.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/pipe.c?h=v4.18#n642

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