This fixes:
```
systemd[1]: Started VictoriaMetrics time series database.
victoria-metrics[379550]: 2021-08-04T19:33:39.833Z panic VictoriaMetrics/lib/storage/partition.go:954 FATAL: unrecoverable error when merging small parts in the partition "/var/lib/victoriametrics/data/small/2021_08": cannot open source part for merging: cannot open metaindex file in stream mode: cannot open file "/var/lib/victoriametrics/data/small/2021_08/1228_1228_20210804184120.712_20210804184121.899_16982E83CD7A763A/metaindex.bin": open /var/lib/victoriametrics/data/small/2021_08/1228_1228_20210804184120.712_20210804184121.899_16982E83CD7A763A/metaindex.bin: too many open files
victoria-metrics[379550]: panic: FATAL: unrecoverable error when merging small parts in the partition "/var/lib/victoriametrics/data/small/2021_08": cannot open source part for merging: cannot open metaindex file in stream mode: cannot open file "/var/lib/victoriametrics/data/small/2021_08/1228_1228_20210804184120.712_20210804184121.899_16982E83CD7A763A/metaindex.bin": open /var/lib/victoriametrics/data/small/2021_08/1228_1228_20210804184120.712_20210804184121.899_16982E83CD7A763A/metaindex.bin: too many open files
victoria-metrics[379550]: goroutine 629 [running]:
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/logger.logMessage(0xbb3ea1, 0x5, 0xc001113800, 0x1e7, 0x4)
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/logger/logger.go:270 +0xc69
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/logger.logLevelSkipframes(0x1, 0xbb3ea1, 0x5, 0xbe3f8b, 0x4b, 0xc000bb3f88, 0x2, 0x2)
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/logger/logger.go:138 +0xd1
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/logger.logLevel(...)
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/logger/logger.go:130
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/logger.Panicf(...)
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/logger/logger.go:126
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/storage.(*partition).smallPartsMerger(0xc0014d7980)
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/storage/partition.go:954 +0x145
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/storage.(*partition).startMergeWorkers.func1(0xc0014d7980)
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/storage/partition.go:933 +0x2b
victoria-metrics[379550]: created by github.com/VictoriaMetrics/VictoriaMetrics/lib/storage.(*partition).startMergeWorkers
victoria-metrics[379550]: github.com/VictoriaMetrics/VictoriaMetrics/lib/storage/partition.go:932 +0x6c
systemd[1]: victoriametrics.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
systemd[1]: victoriametrics.service: Failed with result 'exit-code'.
systemd[1]: victoriametrics.service: Consumed 587ms CPU time, received 6.5K IP traffic, sent 1.7K IP traffic.
systemd[1]: victoriametrics.service: Scheduled restart job, restart counter is at 2064.
systemd[1]: Stopped VictoriaMetrics time series database.
systemd[1]: victoriametrics.service: Consumed 587ms CPU time, received 6.5K IP traffic, sent 1.7K IP traffic.
systemd[1]: Starting VictoriaMetrics time series database...
```
As the only consequence of isSystemUser is that if the uid is null then
it's allocated below 500, if a user has uid = something below 500 then
we don't require isSystemUser to be set.
Motivation: https://github.com/NixOS/nixpkgs/issues/112647
Although the quotes here aren't necessary, they may be if a user cargo-cults this example with a database name with hyphens (or other "unusual" characters).
