[cpio] Fix calculation of name lengths in CPIO headers

[cpio] Fix calculation of name lengths in CPIO headers

Commit 12ea8c4 ("[cpio] Allow for construction of parent directories
as needed") introduced a regression in constructing CPIO archive
headers for relative paths (e.g. simple filenames with no leading
slash).

Fix by counting the number of path components rather than the number
of path separators, and add some test cases to cover CPIO header
construction.

Signed-off-by: Michael Brown <mcb30@ipxe.org>