New @-codes: MSG_AGE (the age of the message in days, months, etc.)

@-codes can now be center-justified by using the *new* -C suffix (like -L/R).
Centered expanded values are not truncated to fit the @-code length however
(that may change).
SMB_AREAS and SMB_AREA_DESC @-codes now display "Email" and "Personal Email"
rather than a blank/empty string when the current message base is the mail base.