daemon: sysroot: Avoid double-/ when creating sysroot paths in OCaml

Previously calling 'sysroot_path "/dev"' for example would return the
string "/sysroot//dev".  While this is not wrong, it confuses some
external programs (hello, setfiles), and it's not very "clean".  Be a
bit more careful to avoid doubling the '/' character in the common case.

(cherry picked from commit 1e0099671a)

daemon/sysroot.ml

@@ -20,4 +20,8 @@ open Std_utils
 
 external sysroot : unit -> string = "guestfs_int_daemon_sysroot"
 
-let sysroot_path path = sysroot () // path
+let sysroot_path path =
+  let sysroot = sysroot () in
+  if path = "" then sysroot
+  else if path.[0] = '/' then sysroot ^ path
+  else sysroot // path

daemon/sysroot.mli

@@ -22,4 +22,4 @@ val sysroot : unit -> string
     in default. *)
 
 val sysroot_path : string -> string
-(** Equivalent to calling [sysroot () // path] *)
+(** Prepend [path] parameter with the sysroot. *)