Ensure systemd-resolved binary is available at runtime

Debian Unstable (and Fedora) has split systemd-resovled into a separate
package which enables systemd-resolved as the default name resolver at
install-time. Because of this, we cannot simply depend on systemd-resolved
in the distro packaging as this may cause the default name resolver to
change to systemd-resolved for some users.

If the user does not have system-resolved installed, fakemachine will
currently fails with any action which requires hostname resolution:

    2022/09/19 10:49:09 ==== debootstrap ====
    2022/09/19 10:49:09 Debootstrap | W: Unable to read /etc/apt/apt.conf.d/ - DirectoryExists (2: No such file or directory)
    2022/09/19 10:49:09 Debootstrap | W: Unable to read /etc/apt/apt.conf.d/ - DirectoryExists (2: No such file or directory)
    2022/09/19 10:49:09 Debootstrap | I: Target architecture can be executed
    2022/09/19 10:49:09 Debootstrap | I: Retrieving InRelease
    2022/09/19 10:49:09 Debootstrap | I: Retrieving Release
    2022/09/19 10:49:09 Debootstrap | E: Failed getting release file http://deb.debian.org/debian/dists/stable/Release
    2022/09/19 10:49:09 debootstrap.log | amd64: ok
    2022/09/19 10:49:09 debootstrap.log | wget: unable to resolve host address 'deb.debian.org'
    2022/09/19 10:49:09 debootstrap.log | wget: unable to resolve host address 'deb.debian.org'

Instead of forcing the user to install systemd-resolved, detect whether
the user has the binary installed and bubble up an error earlier if the
binary has not been found.

See: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1020288
See: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1020690
Signed-off-by: Christopher Obbard <[email protected]>

machine.go

@@ -652,6 +652,7 @@ func (m *Machine) startup(command string, extracontent [][2]string) (int, error)
 
 	/* Add additional binary locations to $PATH */
 	os.Setenv("PATH", os.Getenv("PATH")+":"+prefix+"/sbin")
+	os.Setenv("PATH", os.Getenv("PATH")+":"+prefix+"/lib/systemd")
 
 	/* Search for busybox binary */
 	busybox, err := exec.LookPath("busybox")
@@ -663,6 +664,11 @@ func (m *Machine) startup(command string, extracontent [][2]string) (int, error)
 		return -1, err
 	}
 
+	/* Ensure systemd-resolved binary is available */
+	if _, err := exec.LookPath("systemd-resolved"); err != nil {
+		return -1, err
+	}
+
 	/* Amd64 dynamic linker */
 	err = w.CopyFile("/lib64/ld-linux-x86-64.so.2")
 	if err != nil {