efivars: retry open and read operations

On my laptop (Lenovo X1carbo 4th) I very occasionally see test-boot-timestamps
fail with this tb:

262/494 test-boot-timestamps                    FAIL    0.7348453998565674 s (killed by signal 6 SIGABRT)

08:12:48 SYSTEMD_LANGUAGE_FALLBACK_MAP='/home/zbyszek/src/systemd/src/locale/language-fallback-map' SYSTEMD_KBD_MODEL_MAP='/home/zbyszek/src/systemd/src/locale/kbd-model-map' PATH='/home/zbyszek/src/systemd/build:/home/zbyszek/.local/bin:/usr/lib64/qt-3.3/bin:/usr/share/Modules/bin:/usr/condabin:/usr/lib64/ccache:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/home/zbyszek/bin:/var/lib/snapd/snap/bin' /home/zbyszek/src/systemd/build/test-boot-timestamps
--- stderr ---
Failed to read $container of PID 1, ignoring: Permission denied
Found container virtualization none.
Failed to get SystemdOptions EFI variable, ignoring: Interrupted system call
Failed to read ACPI FPDT: Permission denied
Failed to read LoaderTimeInitUSec: Interrupted system call
Failed to read EFI loader data: Interrupted system call
Assertion 'q >= 0' failed at src/test/test-boot-timestamps.c:84, function main(). Aborting.

Normally it takes ~0.02s, but here there's a slowdown to 0.73 and things fail with EINTR.
This happens only occasionally, and I haven't been able to capture a strace.

It would be to ignore that case in test-boot-timestamps or always translate
EINTR to -ENODATA. Nevertheless, I think it's better to retry, since this gives
as more resilient behaviour and avoids a transient failure.

See
https://github.com/torvalds/linux/blob/master/fs/efivarfs/file.c#L75
and
https://github.com/torvalds/linux/commit/bef3efbeb897b56867e271cdbc5f8adaacaeb9cd.

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