popt: update to 1.18

popt: update to 1.18

Changes from popt 1.16:
- fix an ugly and ancient security issue with popt failing to drop privileges on alias exec from a SUID/SGID program
- perform rudimentary sanity checks when reading in popt config files
- collect accumulated misc fixes (memleaks etc) from distros
- convert translations to utf-8 encoding
- convert old postscript documentation to pdf
- dust off ten years worth of autotools sediment
- reorganize and clean up the source tree for clarity
- remove the obnoxious splint annotations from the sources

Switch to new mirror:
http://ftp.rpm.org/popt/releases/

Switch URL to:
https://github.com/rpm-software-management/popt

Signed-off-by: Nick Hainke <vincent@systemli.org>