Fall back on SetNamedSecurityInfo if SetFileSecurity fails in acl_set_file.

Fall back on SetNamedSecurityInfo if SetFileSecurity fails in acl_set_file.

 src/w32.c (set_named_security_info): New function.
 (acl_set_file): Fall back on set_named_security_info if
 set_file_security fails.  Fixes rare failures in backups.
 (g_b_init_set_named_security_info_w)
 (g_b_init_set_named_security_info_a): New static variables.
 (globals_of_w32): Initialize them to zero.
 (set_named_security_info): Set them to non-zero if the
 corresponding API is available.
 (SetNamedSecurityInfoW_Proc, SetNamedSecurityInfoA_Proc): New
 function typedefs.