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
Revert "Revert "Reenable support for non-PIE executables""

* Conditionally revert under TARGET_NEEDS_NON_PIE_SUPPORT flag

This reverts commit 76e289c026f11126fc88841b3019fd5bb419bb67.

Change-Id: I0c64f219c488f5a507b4e0d651b46f71d1c6b5be