Replace symlink after removing previous symlink

[mjwoods]

Resolves issue #456.

With this change, mirror -R can replace symlinks over sftp.

One side-effect is that ln -s will also replace symlinks, when this would normally be an error.

A better solution would allow mirror -R to replace symlinks without changing the behaviour of ln -s. Any suggestions?

[mjwoods]

The travis-ci build failure seems unrelated to the changes in this PR. The error indicates a missing header file, as follows:

module.cc:30:24: fatal error: configmake.h: No such file or directory

[lavv17]

I don't like removing the old file without knowing it's a symlink first. Mirror should itself remove the old symlink if necessary.