Introduce make_unique_xstrndup

This adds a new make_unique_xstrndup function, which is the "n"
analogue of make_unique_xstrdup.  It also updates a couple existing
places to use this function.

gdbsupport/gdb_unique_ptr.h

@@ -64,4 +64,13 @@ make_unique_xstrdup (const char *str)
   return gdb::unique_xmalloc_ptr<char> (xstrdup (str));
 }
 
+/* Dup the first N characters of STR and return a unique_xmalloc_ptr
+   for the result.  The result is always \0-terminated.  */
+
+static inline gdb::unique_xmalloc_ptr<char>
+make_unique_xstrndup (const char *str, size_t n)
+{
+  return gdb::unique_xmalloc_ptr<char> (xstrndup (str, n));
+}
+
 #endif /* COMMON_GDB_UNIQUE_PTR_H */