ssh/version.scm: Add new procedures

* ssh/version.c (get-libssh-version): Rename to `%get-libssh-version'.
  Return raw libssh version string.
* ssh/version.scm (%get-libssh-version, get-crypto-library)
  (zlib-support?): New procedures.
* doc/api-version.texi: Add description of `%get-libssh-version',
  `get-crypto-library' and `zlib-support?'.
* NEWS: Update.