Fix up the variable names. Also avoid the messy logic of checking whether the
label and context collide with the normal key expansion ones in the face of
adverserial inputs. Make that the caller's responsibility, just as it's already
the caller's responsibility to ensure that different calls don't overlap.  (The
label should be a constant string in an IANA registry anyway.)

Change-Id: I062fadb7b6a18fa946b883be660ea9b3f0f6277c
Reviewed-on: https://boringssl-review.googlesource.com/4216
Reviewed-by: Adam Langley <agl@google.com>