cp: always detect copy-into-self: avoid infloop w/large PATH_MAX

cp: always detect copy-into-self: avoid infloop w/large PATH_MAX

When running the erroneous command, cp -rl A D D, and depending on the
structure of directories A and D and the file system type (because that
changes order of dir. entry traversal), cp would sometimes fail to
detect that D was being copied into D, and would create D/D/D/D/D/...
until it hit PATH_MAX or exhausted some resource.
I noticed this via the occasional failure of the cp/into-self test
when run using a ZFS file system.  It is occasional because the bug
is dependent on the order in which directory entries are traversed,
and that is apparently indeterminate with ZFS.
Technically, with the current recursive implementation, there is no
risk of an infinite loop, due to stack limitations, but with an
eventual fts-based implementation, it might have iterated until
disk space or inodes are exhausted.
* src/copy.c (copy_dir): Avoid copy-into-self interminable loop on
systems with large PATH_MAX.  On other systems, diagnose the copy-into-
self error consistently.  Handle the parameter,
first_dir_created_per_command_line_arg, correctly when there are two
or more sub-directories.