readlink: GNU compatibility

[niyaznigmatullin]

• Added checking for "Not a directory" error when using -e option and the given path is not a directory
• Added checking for "Not a directory" error when using -f option and the parent of the given path is not a directory
• Added tests for readlink and realpath for using trailing slashes
• Fixed behavior when both --no-newline and --zero were used, it shouldn't print the delimiter even if it's --zero.
• Added tests for delimiter options usage

Tests readlink/can-e.sh, readlink/can-f.sh and readlink/multi.sh should pass now

[sylvestre]

Indeed, bravo:

Warning: Congrats! The gnu test tests/readlink/can-e is no longer failing!
Warning: Congrats! The gnu test tests/readlink/can-f is no longer failing!
Warning: Congrats! The gnu test tests/readlink/multi is no longer failing!