Improve error diagnostics of mmap_full_file().

Improve error diagnostics of mmap_full_file().

On mmap errors, this function used to return the error code it got
from mmap(). Unfortunately, in at least two cases, this results in
rather obscure error messages:

	$ para_afh -i .
	main: No such device
	$ touch foo; para_afh -i foo
	main: Invalid argument

This simple patch specifically checks whether the given fd refers
to a directory or an empty file so that the error messages become
more readable:

	$ para_afh -i .
	main: failed to mmap "."
	main: Is a directory
	$ touch foo; para_afh -i foo
	main: failed to mmap "foo"
	main: file is empty