- Make the PROBE_KEYBOARD compile-time option of pxeboot(8) better
resemble the -P option in boot2, i.e., if keyboard isn't present
then boot with both RB_SERIAL and RB_MULTIPLE set.
- Document BOOT_PXELDR_ALWAYS_SERIAL and BOOT_PXELDR_PROBE_KEYBOARD
options of pxeboot(8).
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