MFC r315098,r315106,r315108:

r315098:

Clarify src vs dest path mismatch in :symbolic_link_{absolute,relative}_body

Unfortunately kyua does not omit the path mismatch on failure, so it must be coded
into the error message.

Cache the values, run the test(1) call, then print out the values in an atf_fail
call to emit the required diagnostics to debug why things are failing.

r315106:

Add 3 more testcases demonstrating how install -l sr works

The additional testcases use absolute paths for sources and targets,
as the other testcase which tested -l sr used flat relative paths in
the same directory.

Please note that these testcases do not test -l a -- that's already
addressed in the battery of tests.

r315108:

Restore some of the error message text accidentally removed in r315098

"unexpected symlink contents" is more pedantically correct than
"unexpected symlink".