Created smbmsg_t.flags (run-time flags) and defined MSG_FLAG_HASHED to mark a

message (in memory) as already hashed with smb_hashmsg(), preventing redundant
hashing by smb_addmsghdr().
Using SMB_SUCCESS instead of 0.
Fixed a case where smb_addmsghdr() could return without calling
smb_unlocksmbhdr() if the header was an invalid header length (too large).
smb_addmsghdr() now hashes the header fields if the message has not already
been hashed with smb_hashmsg() before allocating header blocks and
will return a failure (SMB_DUPE_MSG) with detail in smb.last_error if a
duplicate hash was found - new feature of smb_hashmsg().