Support more Syslinux module locations

Support more Syslinux module locations

The current hardcoded directory of Syslinux modules,
/usr/lib/syslinux/modules/bios/, does not exist on ArchLinux. Since
modules are copied with copy_if_exist(), the installer doesn't notify
user when Syslinux modules are missing on the host. Furthermore, when
modules in Grml image are incompatible with Syslinux binary on the host,
the bootloader fails to load graphical menu as described in this bug:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=943878

  Undef symbol FAIL: x86_init_fpu
  Failed to load libcom32.c32
  Failed to load COM32 file vesamenu.c32
  boot:

This change makes grml2usb look for more possible locations on the host
and adds --syslinux-libs option to add another location.