Don't use |BIO_set_foo_buffer_size| when setting the
sizes of the buffers while making buffer pair. Since it
happens in pair.c we know the BIOs are BIO pairs and using
bio_ctrl here complicates setting external buffers. Also
zero out bio_bio_st during construction.

This fixes a problem that would happen if the default buffer
sizes were not set, since buf_externally_allocated was
not yet initialized.

Remove BIO_C_SET_BUFF_SIZE and BIO_CTRL_RESET which are
not used for bio pairs.

Change-Id: I365091d5f44f6f1c5522c325a771bdf03d8fe950
Reviewed-on: https://boringssl-review.googlesource.com/2370
Reviewed-by: Adam Langley <agl@google.com>

Also add functionality for setting external buffers to give the
caller better control of the buffers. This is typical needed if OS
sockets can outlive the bio pair.

Change-Id: I500f0c522011ce76e9a9bce5d7b43c93d9d11457

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)