MFC r319856,r320172,r320173:

r319856:

Add a testcase for ln -sF

The testcase fails today, so mark it with atf_expect_fail: in
particular, the target (B) isn't being unlinked and the documentation
doesn't suggest special handling for directories. Thus, there's either
a doc or an implementation bug in ln(1) that needs to be resolved.

MFC with: r319714, r319854, r319855
PR: 219943

r320172:

ln(1): fix -F behavior

When '-F' option is used, the target directory needs to be unlinked.
Currently, the modified target ("target/source") is being unlinked, and
since it doesn't yet exist, the original target isn't removed.
This is fixed by skipping the block where target is modified to
"target/source" when '-F' option is set.
Hence, a symbolic link (with the same name as of the original target) to
the source_file is produced.

Update the test for ln(1) to reflect fix for option '-F'

PR: 219943

r320173:

Don't expect :sF_flag to fail anymore

While here, also add a check to verify that the link target
is updated in the testcase

MFC with: r320172
PR: 219943