mc_login() should auto-create valid LDAP users

The web login logic auto-creates users in case of BASIC AUTH and LDAP when they don't exist.  However, the SOAP API didn't.

This fix includes:

- When user authenticates using SOAP API, re-use the same auto-creation logic to create the user in case of valid credentials.

- When a user is auto-created, execute post login checks and processing like login count, cookies, tokens, etc.  This fix applies to the web interface code path.

Fixes #15869