======VPOP3.HMAC=====
Added in VPOP3 8.7

This function performs a HMAC (Hash-based Message Authentication Code) algorithm on supplied text using the supplied key

  VPOP3.HMAC(hash algorithm, string, key)

The function takes three parameters:
  - Hash algorithm - "SHA1" or "SHA256"
  - text to hash
  - key to authenticate the hash

The function returns a table of numbers (one byte per table entry) containing the hash value.

If there is an error, the returned table is empty