Add support for signing a file hash

Add support for signing a file hash

In a number of situations, the file hash has already been calculated.
Instead of reading a file to calculate the file hash, read the file hash
from stdin; and instead of writing the signature as an xattr or creating
a .sig file, output the signature as ascii-hex to stdout.

For example, piping the output of sha256sum <pathname> to evmctl would
display the original sha256 output with the file signature appended.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>