Update/fix prototypes in fips.h

misleading/obsolete comment.

FIPS capable: i.e. FIPS module is supplied externally.

New function to lookup digests by NID in module.

Minor optimisation: if supplied hash is NULL to FIPS RSA functions and
we are using PKCS padding get digest NID from otherwise unused saltlen
parameter instead.

applications.

Initialise update function for bad digest inits.

digest in use: this caused the PRF to fail for an odd premaster secret
length.