crypto: sig - Introduce sig_alg backend

Commit 6cb8815f41 ("crypto: sig - Add interface for sign/verify")
began a transition of asymmetric sign/verify operations from
crypto_akcipher to a new crypto_sig frontend.

Internally, the crypto_sig frontend still uses akcipher_alg as backend,
however:

   "The link between sig and akcipher is meant to be temporary.  The
    plan is to create a new low-level API for sig and then migrate
    the signature code over to that from akcipher."
    https://lore.kernel.org/r/ZrG6w9wsb-iiLZIF@gondor.apana.org.au/

   "having a separate alg for sig is definitely where we want to
    be since there is very little that the two types actually share."
    https://lore.kernel.org/r/ZrHlpz4qnre0zWJO@gondor.apana.org.au/

Take the next step of that migration and augment the crypto_sig frontend
with a sig_alg backend to which all algorithms can be moved.

During the migration, there will briefly be signature algorithms that
are still based on crypto_akcipher, whilst others are already based on
crypto_sig.  Allow for that by building a fork into crypto_sig_*() API
calls (i.e. crypto_sig_maxsize() and friends) such that one of the two
backends is selected based on the transform's cra_type.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/crypto/api-sig.rst

@@ -0,0 +1,14 @@
+Asymmetric Signature Algorithm Definitions
+------------------------------------------
+
+.. kernel-doc:: include/crypto/sig.h
+   :functions: sig_alg
+
+Asymmetric Signature API
+------------------------
+
+.. kernel-doc:: include/crypto/sig.h
+   :doc: Generic Public Key Signature API
+
+.. kernel-doc:: include/crypto/sig.h
+   :functions: crypto_alloc_sig crypto_free_sig crypto_sig_set_pubkey crypto_sig_set_privkey crypto_sig_maxsize crypto_sig_sign crypto_sig_verify

Documentation/crypto/api.rst

@@ -10,4 +10,5 @@ Programming Interface
    api-digest
    api-rng
    api-akcipher
+   api-sig
    api-kpp

Documentation/crypto/architecture.rst

@@ -214,6 +214,8 @@ the aforementioned cipher types:
 
 -  CRYPTO_ALG_TYPE_AKCIPHER Asymmetric cipher
 
+-  CRYPTO_ALG_TYPE_SIG Asymmetric signature
+
 -  CRYPTO_ALG_TYPE_PCOMPRESS Enhanced version of
    CRYPTO_ALG_TYPE_COMPRESS allowing for segmented compression /
    decompression instead of performing the operation on one segment