autogen: 5.18.1 - AutoGen/AutoOpts release update

* fixed char casting issue that shows in UTF-8 files
* fixed installation error for str2init
* fixed failure handling in the usage template
* fix non-character flag "values" for auto-supported options
* various tweaks to make Coverity happy.
* allow the fatal error message functions to be tagged "noreturn"
    and incorporate sysnoreturn.h technology into AutoGen.