New options: --owner-map and --group-map.

* NEWS: Update.
* doc/tar.1: Document --owner-map and --group-map
* doc/tar.texi: Likewise.

* src/map.c: New file.
* src/Makefile.am: Add map.c
* src/common.h (owner_map_read, owner_map_translate)
(group_map_read, group_map_translate): New protos.
* src/create.c (start_header): Use owner_map_translate
and group_map_translate to optionally translate user/group names/ids.
* src/tar.c: New options --owner-map and --group-map.

* tests/map.at: New file.
* tests/Makefile.am: Add map.at
* tests/testsuite.at: Include map.at.

NEWS

@@ -1,4 +1,4 @@
-GNU tar NEWS - User visible changes. 2015-08-03
+GNU tar NEWS - User visible changes. 2015-11-02
 Please send GNU tar bug reports to <bug-tar@gnu.org>
 
 
@@ -22,6 +22,24 @@ This option affects all --files-from options that occur after it in
 the command line.  Its effect is reverted by the
 --no-verbatim-files-from option.
 
+* New options: --owner-map=FILE and --group-map=FILE
+
+These two options provide fine-grained control over what user/group
+names (or IDs) should be mapped when adding files to archive.
+
+For both options, FILE is a plain text file with user or group
+mappings.  Empty lines are ignored.  Comments are introduced with
+# sign (unless quoted) and extend to the end of the corresponding
+line.  Each non-empty line defines translation for a single UID (GID).
+It must consist of two fields, delimited by any amount of whitespace:
+
+     OLDNAME NEWNAME[:NEWID]
+
+OLDNAME is either a valid user (group) name or a ID prefixed with +.  Unless
+NEWID is supplied, NEWNAME must also be either a valid name or a
++ID.  Otherwise, both NEWNAME and NEWID need not be listed in the
+system user database.
+
 * --null option reads file names verbatim
 
 The --null option implies --verbatim-files-from.  I.e. each line