quot: Rewrite -n mode input parser

The existing parser was needlessly complicated and wildly inconsistent
in how it handled invalid input. Rewrite using getline() and treat
invalid input consistently: silently ignore lines that don't begin with
a number, and print a warning if the inode number is out of range.

PR: 290992
MFC after: 1 week
Reviewed by: obrien
Differential Revision: https://reviews.freebsd.org/D53726