keys: add new key-type encrypted

Define a new kernel key-type called 'encrypted'. Encrypted keys are kernel
generated random numbers, which are encrypted/decrypted with a 'trusted'
symmetric key. Encrypted keys are created/encrypted/decrypted in the kernel.
Userspace only ever sees/stores encrypted blobs.

Changelog:
- bug fix: replaced master-key rcu based locking with semaphore
  (reported by David Howells)
- Removed memset of crypto_shash_digest() digest output
- Replaced verification of 'key-type:key-desc' using strcspn(), with
  one based on string constants.
- Moved documentation to Documentation/keys-trusted-encrypted.txt
- Replace hash with shash (based on comments by David Howells)
- Make lengths/counts size_t where possible (based on comments by David Howells)
  Could not convert most lengths, as crypto expects 'unsigned int'
  (size_t: on 32 bit is defined as unsigned int, but on 64 bit is unsigned long)
- Add 'const' where possible (based on comments by David Howells)
- allocate derived_buf dy...