If you simply specify the user name, curl will prompt for a password.
+The user name and passwords are split up on the first colon, which makes it
+impossible to use a colon in the user name with this option. The password can,
+still.
+
If you use an SSPI-enabled curl binary and perform NTLM authentication, you
can force curl to select the user name and password from your environment by
specifying a single colon with this option: "-u :".