libzfs: run_process: set O_NONBLOCK on lines pipe

Without this, we can deadlock: the child is stuck writing to the pipe,
and we are stuck waiting on the child

With this, we the child fills up the pipe (a few hundred kBish)
and starts getting EAGAINs, which allows it to either crash
or ignore them

libzfs_run_process_get_stdout*() is used only by zpool -c scripts,
which output short runs of K=V pairs, so the likelihood of losing
legitimate data there is relatively low

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12082