proc: avoid stdio deadlocks

proc: avoid stdio deadlocks

When a request handler accepting post data is too slow in consuming stdin,
uhttpd might deadlock with the master process stuck in a blocking write()
to the child and the child stuck with a blocking write() to the master.

Avoid this issue by putting the master side write end of the child pipe
into nonblocking mode right away and by raising the data_blocked flag
when attempts to write to the child yield EAGAIN.

Setting the flag ensures that client_poll_post_data() does not immediately
trigger a write attempt again, which effectively yields the master write
cycle so that the relay ustream has a chance to consume output of the
client process, thus solving the deadlock.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>