fsearch: write only found files to stdout and use implicit file offset

In the emper-fs-eval I would like to validate the output of the fsearch variants against a list of known files. Not writing any log messages to stdout make that easier.

Use -1 as the offset of the write to stdout. Passing offset -1 means to use the file's implicit offset just like write(2). When using a tty as stdout offset 0 is fine but when redirecting stdout to a file 0 will cause that previously found paths will always be overwritten.