update to go1.18, and use strings.Cut

[thaJeztah]

*: update minimum go version to go1.18

This allows us to start using some new features, like strings.Cut

mount: MergeTmpfsOptions : use strings.Cut

it's faster, and uses less allocations.

mountinfo: add "toInt()" utility

mountinfo: GetMountsFromReader: use strings.Cut

It's faster, and reduces 410 allocations (1976 -> 1566).

Before:

go test -v -test.benchmem -count=10 -run ^$ -bench BenchmarkParseMountinfo .
go: downloading golang.org/x/sys v0.1.0
goos: linux
goarch: arm64
pkg: github.com/moby/sys/mountinfo
BenchmarkParseMountinfo
BenchmarkParseMountinfo-10        4405    283442 ns/op  245426 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4180    258441 ns/op  245425 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4146    258770 ns/op  245425 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4180    259924 ns/op  245425 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4162    263537 ns/op  245425 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4218    261200 ns/op  245425 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4746    259271 ns/op  245425 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4362    265330 ns/op  245425 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4454    263110 ns/op  245425 B/op    1976 allocs/op
BenchmarkParseMountinfo-10        4134    266847 ns/op  245425 B/op    1976 allocs/op
PASS
ok  github.com/moby/sys/mountinfo 11.653s

After

go test -v -test.benchmem -count=10 -run ^$ -bench BenchmarkParseMountinfo .
go: downloading golang.org/x/sys v0.1.0
goos: linux
goarch: arm64
pkg: github.com/moby/sys/mountinfo
BenchmarkParseMountinfo
BenchmarkParseMountinfo-10        4544    262155 ns/op  225746 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4447    266108 ns/op  225745 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4522    246329 ns/op  225745 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4311    249786 ns/op  225745 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4612    250989 ns/op  225745 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4588    249702 ns/op  225745 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4342    247774 ns/op  225745 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4530    246963 ns/op  225745 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4690    248594 ns/op  225745 B/op    1566 allocs/op
BenchmarkParseMountinfo-10        4455    254278 ns/op  225745 B/op    1566 allocs/op
PASS
ok  github.com/moby/sys/mountinfo 11.660s

mountinfo: GetMountsFromReader: inline some assignments