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, 1 insertions, 0 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 85f12d65a..06ab61527 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -474,6 +474,7 @@ vdir_SOURCES = ls.c ls-vdir.c
 id_SOURCES = id.c group-list.c
 groups_SOURCES = groups.c group-list.c
 ls_SOURCES = ls.c ls-ls.c
+ln_SOURCES = ln.c relpath.c relpath.h
 chown_SOURCES = chown.c chown-core.c
 chgrp_SOURCES = chgrp.c chown-core.c
 kill_SOURCES = kill.c operand2sig.c