* util/i386/efi/grub-install.in: Allow `grub_probe --target=partmap'
invocation to fail, in order to support partition-less media.
* util/i386/pc/grub-install.in: Likewise.
* util/powerpc/ieee1275/grub-install.in: Use grub-probe to determine
which fs or partmap modules are needed (akin to its sister scripts).
Also use grub-probe to get rid of unportable /proc/mounts check.
Print the same informational message that the other scripts do, before
exitting.
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