(Imported from upstream's 2747d73c1466c487daf64a1234b6fe2e8a62ac75.)

Also fix up some stylistic issues in conf.c and clarify empty case in
documentation.

Change-Id: Ibacabfab2339d7566d51db4b3ac4579aec0d1fbf
Reviewed-on: https://boringssl-review.googlesource.com/3023
Reviewed-by: Adam Langley <agl@google.com>