nss_files: Allocate nscd file registration data on the heap

This is only needed if nss_files is loaded by nscd.

Before:

   text	   data	    bss	    dec	    hex	filename
    767	      0	  24952	  25719	   6477	nss/files-init.os

After:

   text	   data	    bss	    dec	    hex	filename
    666	      0	      0	    666	    29a	nss/files-init.os

Using PATH_MAX bytes unconditionally for the directory name
is wasteful, but fixing that would constitute another break
of this semi-public ABI.  (The other issue is that with
symbolic links, an arbitrary set of parent directories may need
watching, not just a single one.)

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>