luci2-io-helper: bugfix buckup script read timeout

Reading files from stdin will block for ever. The uhttpd is killing the
backup process after script_timeout.

Switching read to non blocking mode and add a waitpid for the slave
process does not end in a script_timeout anymore.

Signed-off-by: Florian Eckert <Eckert.Florian@googlemail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

Fixes build errors with external toolchains:

[ 33%] Building C object CMakeFiles/cgi-io.dir/main.c.o
/home/florian/dev/openwrt/trunk/build_dir/target-mipsel-unknown-linux-gnu_glibc/cgi-io/main.c:30:21:
fatal error: libubus.h: No such file or directory
 #include <libubus.h>
                     ^
compilation terminated.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Signed-off-by: John Crispin <blogic@openwrt.org>