dirname: split into dirname-lgpl

dirname: split into dirname-lgpl

dirname.h is proving quite useful from multiple LGPL modules,
given the number of trailing slash bugs fixed in syscalls;
only the calls to xalloc_die need to remain GPL.

* modules/dirname-lgpl: New module.
* modules/dirname (Files): Move library-safe files into
dirname-lgpl.
(Depends-on): Add dirname-lgpl.
(configure.ac): Declare witness.
* modules/double-slash-root (License): Relax license.
* lib/dirname.h (base_name, dir_name): Only declare when using GPL
module.
* lib/dirname.c (dir_len, mdir_name): Move...
* lib/dirname-lgpl.c: ...into new file.
* lib/basename.c (last_component, base_len): Move...
* lib/basename-lgpl.c: ...into new file.
* m4/dirname.m4 (gl_DIRNAME_LGPL): New macro.
(gl_DIRNAME): Use it.
* MODULES.html.sh (Enhancements for POSIX:2008 functions):
Mention new module.
* modules/dirname-tests (Depends-on): Add progname.
* tests/test-dirname.c (program_name): Delete.

Signed-off-by: Eric Blake <ebb9@byu.net>