Created smb_updatemsg() - used to safely update the header and index records

of a message.
smb_putmsg() now calls smb_init_idx() automatically to initialize or
re-synchronize the index fields with the header fields - no longer need to call
smb_init_idx() from smb_addmsg() and other places (e.g. fixsmb).
Moved smb_init_idx() from smbhash.c to smblib.c.