feat: implement agent process management (#9461)

- An opt-in feature has been added to the agent to allow
   deprioritizing non coder-related processes for CPU by setting their
   niceness level to 10.
- Opting in to the feature requires setting CODER_PROC_PRIO_MGMT to a non-empty value.

agent/agentproc/syscaller_unix.go

@@ -0,0 +1,42 @@
+//go:build linux
+// +build linux
+
+package agentproc
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+	"golang.org/x/xerrors"
+)
+
+func NewSyscaller() Syscaller {
+	return UnixSyscaller{}
+}
+
+type UnixSyscaller struct{}
+
+func (UnixSyscaller) SetPriority(pid int32, nice int) error {
+	err := unix.Setpriority(unix.PRIO_PROCESS, int(pid), nice)
+	if err != nil {
+		return xerrors.Errorf("set priority: %w", err)
+	}
+	return nil
+}
+
+func (UnixSyscaller) GetPriority(pid int32) (int, error) {
+	nice, err := unix.Getpriority(0, int(pid))
+	if err != nil {
+		return 0, xerrors.Errorf("get priority: %w", err)
+	}
+	return nice, nil
+}
+
+func (UnixSyscaller) Kill(pid int32, sig syscall.Signal) error {
+	err := syscall.Kill(int(pid), sig)
+	if err != nil {
+		return xerrors.Errorf("kill: %w", err)
+	}
+
+	return nil
+}