eventscript: clean up forked handler event code

eventscript: clean up forked handler event code

Write the whole int through the pipe, rather than quietly cutting it
off.  Also, use -2 as the result if the read fails; -1 comes from many
paths if the child fails before running the script.

Add a comment about why we don't need to check the write.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>