Changed name of SMB_BAD_HDR_LEN to SMB_ERR_HDR_LEN.
bugfix: smb_addmsghdr() checks header length before opening sha file.
smb_hfield() now calculates and maintains hdr.length to keep from exceeding
SMB_MAX_HDR_LEN and returns SMB_ERR_HDR_LEN if the new header field would put
the total header length over the max.