Narrow BIO_read and BIO_write error values

Instead of returning arbitrary negative values, always return -1, even
if the callback returns an arbitrary value. Also BIO_write should return
-1, not 0, on error. (BIO_read returns 0 to signal EOF, not error, so
that one is left alone.)

This is in preparation for BIO_read_ex and BIO_write_ex, which are
unable to return such arbitrary values.

Update-Note: BIO_read and BIO_write now each only have one error return.
Test runs suggest no code is relying on the distinction between all the
different return values for error.

Fixed: 42290372
Change-Id: I3385df2f996444d15b2f6918318cae6c78bc7b3a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/96108
Reviewed-by: Lily Chen <chlily@google.com>
Commit-Queue: David Benjamin <davidben@google.com>