match mount points on a segment boundary in handle_file_request

[metsw24-max]

Mount point matched mid-segment in handle_file_request

The mount point is compared as a plain byte prefix, so a mount of /mount also serves /mountdir/... out of the same directory; with the pairing the ServerTest fixture already uses (/ on ./www, /mount on ./www2), GET /mountdir/test.html answers 200 with the contents of ./www2/dir/test.html. Nothing escapes the base directory since is_valid_path still rejects traversal, but the mounted tree becomes reachable under URLs outside the prefix the mount declares, so a reverse proxy location, auth filter or cache rule keyed on /mount/ quietly stops covering files it was meant to guard.

Matching now has to land on a segment boundary. A mount point that already ends in /, the root mount included, keeps its current behaviour, and set_mount_point guarantees the value is non-empty and starts with /.

[yhirose]

@metsw24-max thanks for the excellent contribution!