Merge additional testcases and improvements to bin/ls/ls_tests from

Merge additional testcases and improvements to bin/ls/ls_tests from
^/user/ngie/more-tests.

- Additional testcases added:
-- ls -D
-- ls -F
-- ls -H
-- ls -L
-- ls -R
-- ls -S
-- ls -T
-- ls -b
-- ls -d
-- ls -f
-- ls -g
-- ls -h
-- ls -i
-- ls -k
-- ls -l
-- ls -m
-- ls -n
-- ls -o
-- ls -p
-- ls -q/ls -w
-- ls -r
-- ls -s
-- ls -t
-- ls -u
-- ls -y
- Socket file creation is limited to the ls -F testcase, greatly speeding up
  the test process
- The ls -C testcase was made more robust by limiting the number of columns
  via COLUMNS and by dynamically formulating the columns/lines.
- Add `atf_test_case` before all testcase `head` functions.

X-MFC with: r284388, r288330, r288423
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division