RSASSA-PSS signature scheme as defined by PKCS#11 2.20 using the Botan library

A couple of remarks:

1. This patch implements RSASSA-PSS signature scheme as defined by
PKCS#11 2.20 using the Botan library.
2. I tried to mimic a code style/convention used in the softhsm project,
but could possibly omit something.
3. Due to Botan implementation details MGF1 must be based on the same
hash function as used to digest input data; although this is a
recommended by the PKCS#1 v2.1 specification (see p. 34), there are more
alternatives; nonetheless in case of hash function mismatch a
CKR_BAD_ARGUMENTS value is returned.
4. Since my resources are scarce more testing is appreciated; I tested
only against Botan 1.8.13