Updated copyright notices to include the year 2018.
auth: Removed dead code from oauth2 mechanism
auth: oauth2 mechanism: Fixed implicit boolean expression involving a pointer. Found with Clang -Wstrict-bool.
mech-oauth2: Clarify logging
mech-oauth2: Set user_given=TRUE when username given in OAUTHBEARER
mech-oauth2: Fix gs2-header parsing
auth: Add xoauth2 and oauthbearer mechanisms