Introduced new property value session-full-login to allow the full login string being taken as SIEVE user name.

Comment in file 'mailfilter.properties' was enhanced by:
# Specify which sieve credentials should be use. Two options are allowed here:
# session : login name and password are used from the current session
# session-full-login : full login (incl. context part) name and password are used from the current session
# imapLogin : the login name is taken from the field imapLogin of the current
# user the password is taken from the current session
# mail : use the primary mail address of the user and the password from the
# session
SIEVE_CREDSRC=session