dirname: add library-safe mdir_name

dirname: add library-safe mdir_name

A library-safe dir_name is nice, especially alongside
mfile_name_concat.  Someday, we should rearrange the .o
files so that linking in mdir_name does not suck in
xalloc_die, but for now, the only planned client of
mdir_name (at-func2) is already using xalloc_die.

* lib/dirname.h (mdir_name): New prototype.
* lib/dirname.c (dir_name): Move guts...
(mdir_name): ...to new function that avoids xalloc_die.

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