Fix tagprefix for multiple accounts.
Fix tagprefix for multiple accounts.

It gets very confusing looking at debug logs when all four imapx accounts
are using 'A' as their tagprefix.

gcampax/evolution-data-server,gcampax/evolution-data-server,gcampax/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,gcampax/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server