Implement <pty.h>.

Based on the package/apps/Terminal implementation. I'll switch them over
shortly. This also lets us build the toybox version of netcat.

Change-Id: I0d9bd4d49e9dd6c1ebc7a17ddc955a7693fea664
Signed-off-by: klozz <xxx.reptar.rawrr.xxx@gmail.com>