In sftp.c, the code searches for a colon in the userhost string before parsing
out the host and the user name. The colon should separate the host from the file
to be retrieved, if specified, but because the colon is looked for first, and is
replaced with a NUL, everything before the colon is interpreted to be the
hostname. So:

sftp user:skey@host.com

is recognized as:

sftp user and download a file called "skey@host.com"

which typically results in a "No address associated with nodename" error.

The attached patch places the colon parsing after the user and host name have
been gathered, allowing skey names to work when connecting with sftp.