Fix and improve Opossum check

[drwetter]

Under some circumstances the opossum vulnerability check got stuck because the cat command was waiting for reading from the descriptor. In some case like #2950 this happened when the HTTP head command was incorrectly sent in the first place.

This PR makes sure that the HTTP head is correct and it replaces cat by read in a loop so that the HTTP response is read without being blocked.

Also for http_head_printf() the arguments passed were cleaned up.

What is your pull request about?

• Bug fix
• Improvement
• New feature (adds functionality)
• Breaking change (bug fix, feature or improvement that would cause existing functionality to not work as expected)
• Typo fix
• Documentation update
• Update of other files

If it's a code change please check the boxes which are applicable

• For the main program: My edits contain no tabs, indentation is five spaces and any line endings do not contain any blank chars
• I've read CONTRIBUTING.md and Coding_Convention.md
• I have tested this fix or improvement against >=2 hosts and I couldn't spot a problem
• I have tested this new feature against >=2 hosts which show this feature and >=2 host which does not (in order to avoid side effects) . I couldn't spot a problem
• For the new feature I have made corresponding changes to the documentation and / or to help()
• If it's a bigger change: I added myself to CREDITS.md (alphabetical order) and the change to CHANGELOG.md

[drwetter]

Merging this despite the CI failures -- but the change worked on a Mac.