mount: Allow relative mount destinations on Linux

We tried to make runc enforce abs dest path several times, and always
had to revert it due to some tools not yet doing it. The last occurrence
is this one:
	opencontainers/runc#3944 (comment)

I don't see any reason to force abs dst paths on Linux, as far as I know
there is no security bug nor anything. Let's just relax the spec
wording, matching all the runtimes behavior when the paths is relative,
and be done with it.

Signed-off-by: Rodrigo Campos <[email protected]>

config.md

@@ -68,9 +68,14 @@ For Linux, the parameters are as documented in [mount(2)][mount.2] system call m
 For Solaris, the mount entry corresponds to the 'fs' resource in the [zonecfg(1M)][zonecfg.1m] man page.
 
 * **`destination`** (string, REQUIRED) Destination of mount point: path inside container.
-    This value MUST be an absolute path.
-    * Windows: one mount destination MUST NOT be nested within another mount (e.g., c:\\foo and c:\\foo\\bar).
-    * Solaris: corresponds to "dir" of the fs resource in [zonecfg(1M)][zonecfg.1m].
+    * Linux: This value SHOULD be an absolute path.
+      For compatibility with old tools and configurations, it MAY be a relative path, in which case it MUST be interpreted as relative to "/".
+      Relative paths are **deprecated**.
+    * Windows: This value MUST be an absolute path.
+      One mount destination MUST NOT be nested within another mount (e.g., c:\\foo and c:\\foo\\bar).
+    * Solaris: This value MUST be an absolute path.
+      Corresponds to "dir" of the fs resource in [zonecfg(1M)][zonecfg.1m].
+    * For all other platforms: This value MUST be an absolute path.
 * **`source`** (string, OPTIONAL) A device name, but can also be a file or directory name for bind mounts or a dummy.
     Path values for bind mounts are either absolute or relative to the bundle.
     A mount is a bind mount if it has either `bind` or `rbind` in the options.