[PATCH V2] add basic support for OpenSSL crypto engines

[Grant Zhang]

This patch adds the basic support for OpenSSL crypto engines.

Changes since V1:
- add multiple engine support
- allow default algorithms to be specified for an engine
- remove the support for engine identifier "all" since (a) it is not possible
  to specify default algorithms for all engine and (b) "all" makes it hard to
  figure out what engine does what crypto algorithms.
- address Willy's other comments.