mllib: Add a binding for realpath(3).

This is a copy of the supermin binding:

https://github.com/libguestfs/supermin/tree/master/src realpath*

mllib/unix_utils-c.c

@@ -57,6 +57,7 @@ extern value guestfs_int_mllib_fnmatch (value patternv, value strv, value flagsv
 extern value guestfs_int_mllib_sync (value unitv);
 extern value guestfs_int_mllib_fsync_file (value filenamev);
 extern value guestfs_int_mllib_mkdtemp (value val_pattern);
+extern value guestfs_int_mllib_realpath (value pathv);
 extern value guestfs_int_mllib_statvfs_free_space (value pathv);
 
 /* NB: This is a "noalloc" call. */
@@ -195,6 +196,22 @@ guestfs_int_mllib_mkdtemp (value val_pattern)
   CAMLreturn (rv);
 }
 
+value
+guestfs_int_mllib_realpath (value pathv)
+{
+  CAMLparam1 (pathv);
+  CAMLlocal1 (rv);
+  char *r;
+
+  r = realpath (String_val (pathv), NULL);
+  if (r == NULL)
+    unix_error (errno, (char *) "realpath", pathv);
+
+  rv = caml_copy_string (r);
+  free (r);
+  CAMLreturn (rv);
+}
+
 value
 guestfs_int_mllib_statvfs_free_space (value pathv)
 {

mllib/unix_utils.ml

@@ -56,6 +56,10 @@ module Mkdtemp = struct
     mkdtemp (Filename.concat base_dir (prefix ^ "XXXXXX" ^ suffix))
 end
 
+module Realpath = struct
+  external realpath : string -> string = "guestfs_int_mllib_realpath"
+end
+
 module StatVFS = struct
   external free_space : string -> int64 =
     "guestfs_int_mllib_statvfs_free_space"

mllib/unix_utils.mli

@@ -88,6 +88,11 @@ module Mkdtemp : sig
       [Filename.temp_dir_name] is used. *)
 end
 
+module Realpath : sig
+  val realpath : string -> string
+  (** [realpath(3)] returns the canonicalized absolute pathname. *)
+end
+
 module StatVFS : sig
   val free_space : string -> int64
   (** [free_space path] returns the free space available on the