Added new optional message header field: SMB_COST (in credits)
Added "SMB_" to the names of (previously unused) header field type macros:
SUMMARY, CARBONCOPY, EXPIRATION, and PRIORITY.
Created convenience pointer in smbmsg_t for SUMMARY header field data.
Created convenience values in smbmsg_t for PRIORITY and COST header field data.
Changed smbmsg_t.expiration from a when_t to a ulong (time_t):
expiration dates are always specified in the local timezone.