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