Change for Linux to builtin printf

[drwetter]

Describe your changes

This suggested PR changes the order of which printf is being used in choose_printf() so that Linux uses not /usr/bin/printf . This is for some checks slightly faster.

Also PRINTF is now a tunable parameter which can be set through the enviroment like PRINTF=/usr/bin/printf ./testssl.sh <cmdline>

It doesn't / shouldn't solve the bug #2783. But it might help to reproduce it.

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]

That should not be merged, see #1130