Fixes the following warning when running termux-chroot on Android 8:
proot warning: can't sanitize binding "/property_contexts":
No such file or directory
Allow a command to be specified when running termux-chroot.
Example:
```
$ termux-chroot ls /
bin data dev etc home lib proc property_contexts share storage system tmp usr var vendor
```
This makes it possible to run scripts that include a part that needs to
be run in a chrooted environment, like a compile script.
```
./configure
make
termux-chroot make install
```
Previously it had to be done like this:
```
./configure
make
termux-chroot
cd program
make install
exit
```
