fix(ln): replace existing broken symlinks with -f - #1281

Fixes #1039

ln -sf and ln -f now replace a destination that is a broken symlink. Before, they failed with ln: EEXIST. Real ln -sf replaces it.

src/ln.js checked dest with fs.existsSync, which follows links, so a broken symlink looked absent. The -f unlink was skipped and fs.symlinkSync threw EEXIST. The check now uses common.statNoFollowLinks (lstat). Regular files, directories and valid symlinks are unaffected, because fs.existsSync already returned true for them.

Without -f, a broken-symlink destination now gives Destination file exists, the same as any other existing destination, instead of the raw EEXIST message.

I used try/catch on statNoFollowLinks because rm.js and cp.js use the same pattern to check existence without following links. fs.existsSync(dest) || <lstat>.isSymbolicLink() also works. I can change to that if you prefer it.

The new test -sf option on existing broken symlink in test/ln.js uses the test/resources/badlink fixture. It fails on master with the EEXIST error from the issue and passes with this change.