Add script to run all Perl sources through `perl -c` to ensure no
issues, and run this script via GHA/checksrc in CI.
Fallouts:
- fix two repeated declarations.
- move `shell_quote()` from `testutil.pm` to `pathhelp.pm`, to
avoid circular dependency in `globalconfig.pm`.
Closes#18745
Refreshed, cleaned up, improved and now checks *all* options.
This must have stopped working at some point. gcc-14 least shows these
warnings with this change, not without.
Add test 745 to verify that all options listed in curl.h is also checked
by the typechecker.
This improved checker found almost 30 mistakes in the curl git
repository.
Closes#17143
