License uftpd under the very permissive ISC license

This commit adds the very permissive ISC license to uftpd, with both the
original author and myself as copyright holders.  The original FtpServer
project did not have a license, see xu-wang11/FtpServer#1 for details.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>