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, 16 insertions, 0 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 10be7156b..b7208aa5f 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -9389,6 +9389,22 @@ symbolic link with identical contents; since symbolic link contents
 cannot be edited, any file name resolution performed through either
 link will be the same as if a hard link had been created.
 
+@item -r
+@itemx --relative
+@opindex -r
+@opindex --relative
+Make symbolic links relative to the link location.
+
+Example:
+
+@smallexample
+ln -srv /a/file /tmp
+'/tmp/file' -> '../a/file'
+@end smallexample
+
+@xref{realpath invocation}, which gives greater control
+over relative path generation.
+
 @item -s
 @itemx --symbolic
 @opindex -s