termux-chroot: Specify command to execute

termux-chroot: Specify command to execute
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
```