Support host:guest mount syntax with WORKINGDIR resolution

--mount now accepts <host-path>:<guest-path> (Docker-style). When the
guest path is omitted, the host path is used. Non-absolute guest paths
are resolved against the image WORKINGDIR, which is written to
/slim/workdir at build time and read by slim-init.sh at boot.

Changes:
- qemu.rs: Parse host:guest spec, pass guest path (not host path) on
  the kernel cmdline as slim.mount=<tag>:<base64(guest_path)>
- inject.rs: Accept working_dir param, write /slim/workdir into rootfs
- build.rs: Pass config.working_dir to inject()
- slim-init.sh: Read /slim/workdir, resolve relative guest paths
  against it before mounting
- test.sh: Test host:guest absolute paths (multi-mount) and relative
  guest path resolved against WORKINGDIR

Addresses PR #9 review comment from @hulthe.

src/inject.rs

@@ -100,9 +100,14 @@ fn install_into_rootfs(mount: &str, name: &str, mode: &str, content: &str) -> Re
     Ok(())
 }
 
-/// Inject /slim/init, /slim/exec, and optionally /slim/user into a mounted
-/// container image rootfs.
-pub fn inject(mount_path: &Path, exec_script: &str, user: Option<&str>) -> Result<()> {
+/// Inject /slim/init, /slim/exec, and optionally /slim/user and /slim/workdir
+/// into a mounted container image rootfs.
+pub fn inject(
+    mount_path: &Path,
+    exec_script: &str,
+    user: Option<&str>,
+    working_dir: Option<&str>,
+) -> Result<()> {
     let mount_str = mount_path.to_str().context("mount path is not UTF-8")?;
 
     install_into_rootfs(mount_str, "init", "755", SLIM_INIT)?;
@@ -112,5 +117,9 @@ pub fn inject(mount_path: &Path, exec_script: &str, user: Option<&str>) -> Resul
         install_into_rootfs(mount_str, "user", "644", user)?;
     }
 
+    if let Some(dir) = working_dir.filter(|d| !d.is_empty()) {
+        install_into_rootfs(mount_str, "workdir", "644", dir)?;
+    }
+
     Ok(())
 }