As useless as it might seem, the certificates in PKCS#12 files appear to
always be encrypted with 40-bit RC2. OpenSSL, NSS and Windows are all
the same on this point. Thus, in order to be able to import PKCS#12
files we need RC2 support.

RC2 has deliberately not been added to EVP_get_cipherbynid so that the
linker can drop the RC2 code unless the PKCS#12 functions are actually
called.

Change-Id: I5b2062fdf78cb622a8038c326da01aac8fb58962
Reviewed-on: https://boringssl-review.googlesource.com/1590
Reviewed-by: Adam Langley <agl@google.com>