Mirror how chaseat() works these days: instead of a single toplevel_fd that
serves as both the root (chroot) boundary and the directory that resolution
starts from, path_pick() now takes a separate root_fd and dir_fd. This lets
callers resolve a path relative to a specific directory fd while confining
symlink and absolute-path resolution to a root directory fd.
All existing callers are updated to pass the same fd for both, preserving
their current behaviour.
Lennart Poettering
95a372a514