ln: add the --relative option

With the "--relative --symbolic" options, ln computes the relative
symbolic link for the user.

So, ln works just as cp, but creates relative symbolic links instead
of copying the file.

I miss this feature since the beginning of using ln.

$ tree ./
/
`-- usr
    |-- bin
    `-- lib
        `-- foo
            `-- foo

4 directories, 1 file

$ ln -s -v --relative usr/lib/foo/foo usr/bin/foo
‘usr/bin/foo’ -> ‘../lib/foo/foo’

$ tree ./
/
`-- usr
    |-- bin
    |   `-- foo -> ../lib/foo/foo
    `-- lib
        `-- foo
            `-- foo

4 directories, 2 files

$ ln -s -v --relative usr/bin/foo usr/lib/foo/link-to-foo
‘usr/lib/foo/link-to-foo’ -> ‘foo’

$ tree ./
/
`-- usr
    |-- bin
    |   `-- foo -> ../lib/foo/foo
    `-- lib
        `-- foo
            |-- link-to-foo -> foo
            `-- foo

4 directories, 3 files

* src/Makefile.am: Reference the relpath module.
* src/ln.c (usage): Mention the new option.
(do_link): Call the relative conversion if specified.
(convert_abs_rel): Perform the relative conversion
using the relpath module.
* tests/ln/relative: Add a new test.
* tests/Makefile.am: Reference the new test.
* doc/coreutils.texi: Document the new feature.
* NEWS: Mention the new feature.

1 files changed, 3 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 5b53eb8a0..c9ed18521 100644
--- a/NEWS
+++ b/NEWS
@@ -25,6 +25,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   dd now accepts the conv=sparse flag to attempt to create sparse
   output, by seeking rather than writing to the output file.
 
+  ln now accepts the --relative option, to generate a relative
+  symbolic link to a target, irrespective of how the target is specified.
+
   split now accepts an optional "from" argument to --numeric-suffixes,
   which changes the start number from the default of 0.