Handle EINTR in open and dup calls in urandom.c.

Per review comment in
https://boringssl-review.googlesource.com/#/c/5302/7/crypto/rand/urandom.c

Change-Id: I9c279524a452cb97c60354213cbc6e2aeabe0bfa
Reviewed-on: https://boringssl-review.googlesource.com/5311
Reviewed-by: Adam Langley <agl@google.com>