Various cleanups to the software encryption transform interface.

Various cleanups to the software encryption transform interface.

- Consistently use 'void *' for key schedules / key contexts instead
  of a mix of 'caddr_t', 'uint8_t *', and 'void *'.

- Add a ctxsize member to enc_xform similar to what auth transforms use
  and require callers to malloc/zfree the context.  The setkey callback
  now supplies the caller-allocated context pointer and the zerokey
  callback is removed.  Callers now always use zfree() to ensure
  key contexts are zeroed.

- Consistently use C99 initializers for all statically-initialized
  instances of 'struct enc_xform'.

- Change the encrypt and decrypt functions to accept separate in and
  out buffer pointers.  Almost all of the backend crypto functions
  already supported separate input and output buffers and this makes
  it simpler to support separate buffers in OCF.

- Remove xform_userland.h shim to permit transforms to be compiled in
  userland.  Transforms no longer call malloc/free directly.

Reviewed by:	cem (earlier version)
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D24855