Minor improvements: modern methods, less panic-prone

[michurin]

Minor code update:

• Using r.URL.Query().Get("...") instead of r.URL.Query()["..."] where this is the case
• Using modern strings.Cut instead of strings.SplitN(..., 2)
• Bumping required go version up to 1.18 to enable strings.Cut using
• Fixing panic-prone code w.Header().Set(h[0], strings.TrimSpace(h[1])), where h can have arbitrary length
• Dealing with ROUTES similarly to PORT: check len() of value