Security update for libzypp, zypper (important)

The Software Update Stack was updated to receive fixes and enhancements.

libzypp:

• Adapt to work with GnuPG 2.1.23. (bsc#1054088)
• Support signing with subkeys. (bsc#1008325)
• Enhance sort order for media.1/products. (bsc#1054671)

zypper:

• Also show a gpg key’s subkeys. (bsc#1008325)
• Improve signature check callback messages. (bsc#1045735)
• Add options to tune the GPG check settings. (bsc#1045735)
• Adapt download callback to report and handle unsigned packages.
  (bsc#1038984, CVE-2017-7436)
• Report missing/optional files as ‘not found’ rather than ‘error’.
  (bsc#1047785)