sender: open a module-root-absolute path for a `path = /` module (#897)

sender: open a module-root-absolute path for a `path = /` module (#897)

A daemon module with path=/ makes F_PATHNAME absolute, so the secure_path built
for the content open starts with '/'.  secure_relative_open() rejects an
absolute relpath with EINVAL, so a use-chroot=no daemon with path=/ could not
send any file ('failed to open ...: Invalid argument (22)') -- a regression
from 3.4.2.  Strip leading slashes to a module-relative path; resolution stays
confined beneath module_dir.