Revert "Revert "Add path interposer""

This reverts commit 96c957ae.

Fixes issues on some machines where the socket in TMPDIR ended up with a
unix domain socket pathname over 107 characters long, which Go will
reject due to underlying limitations in the system calls. If this
happens, we'll fall back to opening the directory, then using
/proc/self/fd/#/<file>, or manually creating a similar symlink in /tmp.

Also fixes some issues on Mac where os.Executable returns the symlink
instead of the underlying file, sending a message over a unix domain
socket will block if the reader isn't reading, and sandboxing was
preventing us from running `ps`.

Test: m blueprint_tools
Test: m blueprint_tools on mac
Change-Id: Ib19ccfe10cb0a79f1476fb1d5cd20ed0495be367