Turn password_encryption GUC into an enum.

This makes the parameter easier to extend, to support other password-based
authentication protocols than MD5. (SCRAM is being worked on.)

The GUC still accepts on/off as aliases for "md5" and "plain", although
we may want to remove those once we actually add support for another
password hash type.

Michael Paquier, reviewed by David Steele, with some further edits by me.

Discussion: <CAB7nPqSMXU35g=W9X74HVeQp0uvgJxvYOuA4A-A3M+0wfEBv-w@mail.gmail.com>