Before this patch the script relied on Perl `system()` finding `openssl`
in `PATH`, plus tried to display the full path of `openssl` by using
`command -v` (or `which` on Windows). `command -v` did not work in CI
for unknown reasons. To resolve it, this patch detects `openssl` in
`PATH` manually, displays the detected full path and calls `openssl`
with the detected full path, and stops relying on `system` for this.
It also follows how `sshhelp.pm` is detecting executables. Though this
patch uses Perl `-f` instead of `-e && -d` used there .
Silencing this in CI logs:
```
Can't exec "command": No such file or directory at ../../../tests/certs/genserv.pl line 51.
```
Ref: https://github.com/curl/curl/actions/runs/14145795884/job/39632942668?pr=16865#step:39:108Closes#16868
To remove POSIX shell as an extra dependency for runtests.
Also fix to `chmod 0600` the `.pem` file (was: `.prm`), and apply it
_before_ writing the keys.
Follow-up to 44341e736a#16824Closes#16858
