xargs: fix parsing of empty fields when "xargs -0" is used.
Previously, these fields would be skipped. From Hiltjo Posthuma.

Specify the xargs binary to test using a variable.

While here, do not suppress the command executed by make.

Fix -L/-I processing in -0 mode so that NUL-delimited entries are
treated as "lines". From FreeBSD.

test blank and newline handling in xargs(1) with and without -0 and -L

ok guenther@