'zap' doesn't unmount all mounts
After rebasing off of the master branch here, pmbootstrap zap doesn't unmount all mounts, resulting in a failure:
$ ./pmbootstrap.py zap i
[21:36:54] Unregister qemu binfmt (arm)
[21:36:56] Shutdown complete
[21:36:56] Remove /home/clayton/src/pmbootstrap-out/chroot_native? (y/n) [n]: y
[21:37:04] NOTE: The failed command's output is above the ^^^ line in the logfile: /home/clayton/src/pmbootstrap-out/log.txt
[21:37:04] ERROR: Command failed: % sudo rm -rf /home/clayton/src/pmbootstrap-out/chroot_native
[21:37:04] Run 'pmbootstrap log' for details.
[21:37:04] See also: <https://postmarketos.org/troubleshooting> Here is output from mount after running zap:
$ mount
...
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_native/var/cache/apk type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_native/home/user/.ccache type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_native/var/cache/distfiles type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_native/home/user/git type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_native/home/user/.abuild type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_native/etc/apk/keys type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_native/home/user/packages/user type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
proc on /media/data/clayton/src/pmbootstrap-out/chroot_rootfs_nokia-rx51/proc type proc (rw,nosuid,nodev,noexec,relatime)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_rootfs_nokia-rx51/var/cache/apk type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_rootfs_nokia-rx51/home/user/.ccache type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_rootfs_nokia-rx51/var/cache/distfiles type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_rootfs_nokia-rx51/home/user/git type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_rootfs_nokia-rx51/home/user/.abuild type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_rootfs_nokia-rx51/etc/apk/keys type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_rootfs_nokia-rx51/home/user/packages/user type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
devtmpfs on /media/data/clayton/src/pmbootstrap-out/chroot_native/dev/install type devtmpfs (rw,nosuid,size=16168212k,nr_inodes=4042053,mode=755)
devtmpfs on /media/data/clayton/src/pmbootstrap-out/chroot_native/dev/installp1 type devtmpfs (rw,nosuid,size=16168212k,nr_inodes=4042053,mode=755)
devtmpfs on /media/data/clayton/src/pmbootstrap-out/chroot_native/dev/installp2 type devtmpfs (rw,nosuid,size=16168212k,nr_inodes=4042053,mode=755)
/dev/installp2 on /media/data/clayton/src/pmbootstrap-out/chroot_native/mnt/install type ext4 (ro,relatime,data=ordered)
/dev/installp1 on /media/data/clayton/src/pmbootstrap-out/chroot_native/mnt/install/boot type ext2 (rw,relatime,block_validity,barrier,user_xattr,acl)
/dev/mapper/data on /media/data/clayton/src/pmbootstrap-out/chroot_native/mnt/rootfs_nokia-rx51 type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)