sasl: make login option string override http auth

sasl: make login option string override http auth

- Use http authentication mechanisms as a default, not a preset.

Consider http authentication options which are mapped to SASL options as
a default (overriding the hardcoded default mask for the protocol) that
is ignored if a login option string is given.

Prior to this change, if some HTTP auth options were given, sasl mapped
http authentication options to sasl ones but merged them with the login
options.

That caused problems with the cli tool that sets the http login option
CURLAUTH_BEARER as a side-effect of --oauth2-bearer, because this flag
maps to more than one sasl mechanisms and the latter cannot be cleared
individually by the login options string.

New test 992 checks this.

Fixes https://github.com/curl/curl/issues/10259
Closes https://github.com/curl/curl/pull/12790