mllib: Add unsetenv(3) binding to Unix_utils.

Missing from the OCaml stdlib.  This implementation is the same as the
one in Jane St's Core, except we don't bother to throw an exception on
error.

mllib/unix_utils-c.c

@@ -45,6 +45,7 @@ extern void unix_error (int errcode, char * cmdname, value arg) Noreturn;
 extern value guestfs_int_mllib_dev_t_makedev (value majv, value minv);
 extern value guestfs_int_mllib_dev_t_major (value devv);
 extern value guestfs_int_mllib_dev_t_minor (value devv);
+extern value guestfs_int_mllib_unsetenv (value strv);
 extern int guestfs_int_mllib_exit (value rv) __attribute__((noreturn));
 extern value guestfs_int_mllib_fnmatch (value patternv, value strv, value flagsv);
 extern value guestfs_int_mllib_sync (value unitv);
@@ -73,6 +74,14 @@ guestfs_int_mllib_dev_t_minor (value devv)
   return Val_int (minor (Int_val (devv)));
 }
 
+/* NB: This is a "noalloc" call. */
+value
+guestfs_int_mllib_unsetenv (value strv)
+{
+  unsetenv (String_val (strv));
+  return Val_unit;
+}
+
 /* NB: This is a "noalloc" call. */
 int
 guestfs_int_mllib_exit (value rv)

mllib/unix_utils.ml

@@ -22,6 +22,10 @@ module Dev_t = struct
   external minor : int -> int = "guestfs_int_mllib_dev_t_minor" "noalloc"
 end
 
+module Env = struct
+  external unsetenv : string -> unit = "guestfs_int_mllib_unsetenv" "noalloc"
+end
+
 module Exit = struct
   external _exit : int -> 'a = "guestfs_int_mllib_exit" "noalloc"
 end

mllib/unix_utils.mli

@@ -32,6 +32,11 @@ module Dev_t : sig
   (** minor(3) *)
 end
 
+module Env : sig
+  val unsetenv : string -> unit
+  (** Unset named environment variable, see unsetenv(3). *)
+end
+
 module Exit : sig
   val _exit : int -> 'a
   (** Call _exit directly, ie. do not run OCaml atexit handlers. *)