Rerwrite _encipher() as _process(); def _decipher().

algorithms/encryption/des/src/des.c
	-Generalize `DES_encipher()` into `_process()`, because
	essentially all of the DES algorithm's machinery is identical
	for enciphering/deciphering, with the exception of the order in
	which subkeys are passed to `_expansionPermutation()`.
	-Redefine `DES_encipher()` and define `DES_decipher` as bare
	wrappers around `_process()`; introduce constants `g_DECIPHER`,
	`g_ENCIPHER` to indicate which process is being run.
	-Document `_process()`.