Public API to check for optional feature

[Remi Collet <remi%fedoraproject.org@localhost>]

Hi,

Some recently added features are optional:

* bzip2 compression
* AES encryption
* xz compression


I think it make sense to have some public method
to be able to check if available.

For example, on some archive application, to be
able to offer compression algo choice, among available
ones (else, have to try and check failure code)

PR open https://github.com/nih-at/libzip/pull/152

* zip_compression_method_supported already exists
  and is only changed to be public

* zip_encryption_method_supported added

* ziptools changed to take benefit of this


Feedback / review welcome


Remi


P.S. also planed to be added in PHP zip extension.

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