Support bios.syslinux.eltorito boot mode without bios.syslinux.mbr.
bios.syslinux.mbr does not work without bios.syslinux.eltorito because -isohybrid-mbr requires the El Torito boot image.

Support uefi-x64.systemd-boot.esp boot mode without uefi-x64.systemd-boot.eltorito and vice versa.
If uefi-x64.systemd-boot.eltorito is used without uefi-x64.systemd-boot.esp, the El Torito boot image will be placed in the ISO 9660 file system as before.
Note that an ISO created with only uefi-x64.systemd-boot.eltorito will still be bootable as a "hard disk" on OVMF. OVMF will boot the El Torito image.

This change has the following effect on the partition tables:

- *.eltorito options add El Torito boot catalog entries. MBR and GPT are not affected.
- uefi-x64.systemd-boot.esp creates a protective MBR partition table and a GPT table that includes a Linux filesystem data partition for the ISO 9660 volume, an EFI system partition and a Microsoft basic data partition that maps the 300 KiB padding added by xorriso.
- bios.syslinux.mbr (without uefi-x64.systemd-boot.esp): adds a MBR partition table and maps the ISO 9660 volume as a partition of type 0x83. No GPT is produced.
- bios.syslinux.mbr (with uefi-x64.systemd-boot.esp): adds an second partition in the MBR (after 0xEE) starting from sector 0 to sector 1 with type 0 and marks it as bootable. This violates the GPT specification, but allows some systems to succesfully boot in BIOS mode from GPT.

Fixes archlinux/archiso#49