INT 21 - LANtastic - GET USERNAME ENTRY
AX = 5F83h
BX = login entry index (0-based)
ES:DI -> 16-byte buffer for username currently logged into
Return: CF clear if successful
DL = adapter number (v3+)
CF set on error
AX = error code
Note:   the login entry index corresponds to the value BX used in AX=5F80h

See also: 215F80

215F83 - LANtastic - GET USERNAME ENTRY