x/playground: send SIGQUIT on timeout to provide useful context

[toothrot]

The playground could kill processes in a way that provided the user helpful context instead of Process.Kill().

What version of Go are you using (go version)?

$ go version
go version go1.14 linux/amd64 (playground)

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

package main

func main() {
	for {
	}
}

What did you expect to see?

timeout running program
SIGQUIT: quit
PC=0x458050 m=0 sigcode=0

goroutine 1 [running]:
main.main()
	/tmp/sandbox300849141/prog.go:4 fp=0xc000034788 sp=0xc000034780 pc=0x458050
runtime.main()
	/usr/local/go-faketime/src/runtime/proc.go:203 +0x20e fp=0xc0000347e0 sp=0xc000034788 pc=0x42bf4e
runtime.goexit()
	/usr/local/go-faketime/src/runtime/asm_amd64.s:1373 +0x1 fp=0xc0000347e8 sp=0xc0000347e0 pc=0x453831

rax    0x458050
rbx    0x0
rcx    0x4d0000
rdx    0x47a788
rdi    0xc000060010
rsi    0x0
rbp    0xc0000347d0
rsp    0xc000034780
r8     0x0
r9     0x1
r10    0xc000060000
r11    0x1
r12    0x0
r13    0x40
r14    0x483ecc
r15    0x0
rip    0x458050
rflags 0x246
cs     0x33
fs     0x0
gs     0x0

Program exited: status 2.

What did you see instead?

timeout running program

/cc @bcmills @cagedmantis @dmitshur

[gopherbot]

Change https://golang.org/cl/227652 mentions this issue: sandbox: timeout runsc commands

[toothrot]

We should also clean up usages of CommandContext when we want to send a friendlier signal (golang.org/cl/227351 comment)