quot: Clean up

• Fix numerous style violations.
• Modernize somewhat.
• Don't bother examining errno after calling get_inode(), as it always exits on error.
• Fix confusing wording in the manual page.

The code remains somewhat idiosyncratic, e.g. in its insistance on
counting down rather than up in simple for loops, but in the absence
of comprehensive automated tests, the risk of introducing bugs exceeds
the benefit of rewriting these into more idiomatic forms.

Reviewed by: obiwac
Differential Revision: https://reviews.freebsd.org/D53130

(cherry picked from commit 195b00ec45e55c38da13d1bcd2d7eb6614abec59)