Realpath symlinks handling, solves issue #3669

[niyaznigmatullin]

Fixes issue #3669

• Made better component-wise symlink handling, if any part of the path inside symlink is non-existent it fails now

• Fixed test for dangling symlink which was mistakenly changed to failing, try:

    $ ln -s nonexisting link
    $ realpath link
  

  It should print path to nonexisting as GNU version does

• Fixed test for looping symlink, GNU version does make an error

    $ ln -s 1 2
    $ ln -s 2 3
    $ ln -s 3 1
    $ realpath 1
  

  It should fail with Too many levels of symbolic links

[niyaznigmatullin]

After changing canonicalize function, some cp util usages began to fail, it incorrectly spotted whether it required to copy symlink itself or copy the contents, when checking for errors. So,

• Fixed how to check if source and destination are the same file, using FileInformation
• When copying symlink over symlink with no-dereference option, symlink not always removed (i.e. it was dangling or was a symlink to directory), fixed it
• When destination is symlink, no-dereference is used and source is a regular file it didn't check that destination is dangling (it thought symlink copy is going to take place, but since source is a regular file, contents are copied)
• Added more tests for symlinks, and no-dereference options

Also all tests that contained relative symlinks with / inside the symlink didn't work on Windows, I think that's because Windows itself doesn't support it (mklink ../t a doesn't work with strange message, but mklink ..\t a works correctly), it creates it, but then it can't open the file through symlink. So I changed that in tests when creating relative symlinks all slashes are substituted to a correct separator.

The part of cp with preserving hardlinks used unsafe syscalls, I found out it fails silently in Windows sometimes, I substituted it to using FileInformation, since it does the same.

[sylvestre]

wahou:
bravo!

Warning: Congrats! The gnu test tests/cp/no-deref-link1 is no longer failing!
Warning: Congrats! The gnu test tests/cp/no-deref-link2 is no longer failing!
Warning: Congrats! The gnu test tests/cp/slink-2-slink is no longer failing!
Warning: Congrats! The gnu test tests/ln/relative is no longer failing!
Warning: Congrats! The gnu test tests/misc/readlink-root is no longer failing!