libiberty: add ldirname function

libiberty: add ldirname function

This patch adds a function ldirname to libiberty.  It is implemented
in terms of lbasename.  Basically, given a given pathname, the dirname
part is what is not the basename minus the last directory separator
separating the dirname with the basename.

include/ChangeLog

	* libiberty.h (ldirname): New function declaration.
	(dos_ldirname): Likewise.
	(unix_ldirname): Likewise.

libiberty/ChangeLog

	* ldirname.c: New file.
	* Makefile.in (CFILES): Add ldirname.c.
	(REQUIRED_OFILES): Add ldirname.$(objext).
	(./ldirname.$(objext)): New rule.
	* makefile.vms (OBJS): Add ldirname.obj.
	* configure.com (FILES): Add ldirname.