Fix memory leaks during "tail -r" of an irregular file

• Rewrite r_buf to use standard tail queues instead of a hand-rolled circular linked list. Free dynamic allocations when done.
• Remove an optimization for the case where the file is a multiple of 128KB in size and there is a scarcity of memory.
• Add ATF tests for "tail -r" and its variants.

Reported by: Valgrind
Reviewed by: ngie
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D9067