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: Ia922a100141a67409264b43b937eeca07b21f344
Merge "Implement <pty.h>."
Remove __ARM_HAVE_VFP.

Bug: 18556103
Change-Id: I6d4cc6a1b359ad2df1ce6687fd26f392059f6efd
Merge "Remove __ARM_HAVE_VFP."