Leave Basic auth enabled when NTLM is configured

It turns out people were using the fallback from NTLM to Basic when the
default configuration has NTLM enabled.

It breaks our GSSAPI hacks though, so still disable it in that case.
Add camel_ews_settings_get_auth_mechanism() helper function

The auth-mechanism string is not fun to deal with. It can sometimes
be NULL and sometimes an empty string for NTLM, and I've managed to
screw up even the *simple* cases, comparing for "BASIC" and then "Basic"
in subsequent broken commits (b6ff74a4, 377da623) instead of "PLAIN".

Make it nice and simple. Hard-code this knowledge about the strings in
*one* place, camel_ews_settings_get_auth_mechanism(), and return an
enum. Which in turn will help us to ensure we handle all cases, in
places where that's appropriate (such as in
e_ews_connection_utils_get_without_password())