ip/xfrm: Improve usage text and documentation

ip/xfrm: Improve usage text and documentation

Change ALGO-KEY to ALGO-KEYMAT to make it more obvious that the
keying material might need to contain more than just the key (such
as a salt or nonce value).

List the algorithm names that currently exist in the kernel.

Indicate that for IPComp, the Compression Parameter Index (CPI) is
used as the SPI.

Group the list of mode values by transform protocol.

Signed-off-by: David Ward <david.ward@ll.mit.edu>