Promote libevent to lib/ level and fold ftp-proxy into its parent Makefile.
This allows us to use libevent for other application in the future.

For now libevent is still INTERNALLIB and no shared library is installed.

MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273065 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f