child_process: fix sending utf-8 to child process

In process#send() and child_process.ChildProcess#send(), use 'utf8' as
the encoding instead of 'ascii' because 'ascii' mutilates non-ASCII
input. Correctly handle partial character sequences by introducing
a StringDecoder.

Sending over UTF-8 no longer works in v0.10 because the high bit of
each byte is now cleared when converting a Buffer to ASCII. See
commit 96a314b6 for details.

Fixes #4999 and #5011.