tar: --owner and --group names and numbers

The --owner and --group options now accept operands of the form
NAME:NUM, so that you can specify both symbolic name and numeric
ID for owner and group.  Also, in these options, NAME no longer
needs to be present in the current host's user and group
databases; this implements Debian enhancement request 136231
<http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=136231> reported
by Mark W. Eichin, communicated by Thayne Harbaugh to bug-tar in
<http://lists.gnu.org/archive/html/bug-tar/2011-08/msg00001.html>.
* NEWS, doc/tar.texi (Option Summary, override): Document enhancement.
* src/common.h (group_name_option, owner_name_option): New decls.
* src/create.c (start_header): Don't assume owner and group names
are in current host database.
* src/tar.c (parse_owner_group): New function, for parsing NAME:NUM.
(parse_opt): Use it.
(decode_options): Initialize owner_name_option, group_name_option.
* tests/owner.at: New file, to test this enhancement.
* tests/Makefile.am (TESTSUITE_AT): Add it.
* tests/testsuite.at: Include it.