Add SSL_CTX_set_keylog_bio.

Configures the SSL stack to log session information to a BIO. The intent is to
support NSS's SSLKEYLOGFILE environment variable. Add support for the same
environment variable to tool/client.cc.

Tested against Wireshark 1.12.0.

BUG=393477

Change-Id: I4c231f9abebf194eb2df4aaeeafa337516774c95
Reviewed-on: https://boringssl-review.googlesource.com/1699
Reviewed-by: Adam Langley <agl@google.com>