lib/sysroot: Call `chmod` on overlay dirs when unlocking

lib/sysroot: Call `chmod` on overlay dirs when unlocking

Otherwise, we'll be subject to whatever `umask` is currently. Normally,
processes should respect `umask` when creating files and directories,
but specifically for `ostree admin unlock` (or `rpm-ostree usroverlay`),
this poses a problem since e.g. a `/usr` with mode 0700 will break any
daemon that doesn't run as root and needs to read files under `/usr`,
such as polkitd.

This patch just does a `chmod()` after the `mkdir()`. An alternative
would be to do `umask(0000)` after forking into the child process
that'll call `mount()`, but that'd require also moving the `mkdir()`
calls into there, making for a more intrusive patch.

Closes: #1843
Approved by: cgwalters