First batch of the alphabet.

Change-Id: If4e60f4fbb69e04eb4b70aa1b2240e329251bfa5
Reviewed-on: https://boringssl-review.googlesource.com/4514
Reviewed-by: Adam Langley <agl@google.com>

Including string.h in base.h causes any file that includes a BoringSSL
header to include string.h. Generally this wouldn't be a problem,
although string.h might slow down the compile if it wasn't otherwise
needed. However, it also causes problems for ipsec-tools in Android
because OpenSSL didn't have this behaviour.

This change removes string.h from base.h and, instead, adds it to each
.c file that requires it.

Change-Id: I5968e50b0e230fd3adf9b72dd2836e6f52d6fb37
Reviewed-on: https://boringssl-review.googlesource.com/3200
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

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.)