meson: support both separate and merged sbin-bin directories

meson: support both separate and merged sbin-bin directories

Follow-up for ba7f4ae6178309dc937e10cf7dce0eca9dafb8de.

By default, we detect if the real root has a separate /usr/sbin directory, but
this can be overrides with -Dsplit-bin=true|false. The check assumes that
/usr/sbin is split if it is not a symlink, so it'll return a false negative
with some more complicated setups. But that's OK, in those cases this should be
configured explicitly.

This will copy the structure of the directories in the root file system to
$DESTDIR. If a directory is a directory in $DESTDIR but a symlink in the root
file system, this script will fail. This means that it's not possible to reuse
a $DESTDIR from between ba7f4ae61 and this patch.