Read PKCS 12 files

[jeroen]

To do:

• [x] parser
• [x] writer / generator
• [ ] debug cert bundle reader
• [ ] verify certs
• [x] add unit tests

See also https://github.com/NCEAS/PKIplus/issues/3. Resources:

• Example code
• Test certs
• UseR video

[jeroen]

First pass: https://github.com/jeroenooms/openssl/commit/5c513cc73646ef0431dd0103769530c0281e2e29. Not tested the CA parser yet because I can't find any example p12 files with a CA bundle.

[jeroen]

Added some unit tests: https://github.com/jeroenooms/openssl/blob/master/tests/testthat/test_pkcs12.R