Save/restore errno in SIGCHLD handler.

The SIGCHLD handler can set the errno, which can change the errno found
on the thread on which the signal handler is invoked. Most of the time
this is harmless, but it's possible that if the main thread was making
a system call, and allows certain failures based on errno, that logic
can fail. The fix is to save/restore errno in the handler.

Bug: 23572286
Bug: 23689391
Change-Id: I4542fd60cc1398ce0a8902d8df98a3d089fb6efe