uucore(quoting_style): Improve quoting style handling

[RenjiSann]

This MR introduces constants for quoting styles to simplify the declaration of quoting style variables throughout the codebase.

It also changes the interface of escape_name family of function to take QuotingStyle instead of &QuotingStyle as a parameter, because the struct implements Copy anyway

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[sylvestre]

much better!