Fix edge cases when specifying the user to run as.

* check the username fits the format expected by adduser;
 * don't allow root;
 * if the user exists, don't try to create it; usually, this is
   harmless, but it fails if the user exists with a UID outside the
   normal range for system users.

Fixes #24.

debian/dump1090-mutability.templates

@@ -332,3 +332,7 @@ Type: error
 Template: dump1090-mutability/invalid-is_valid_gain
 Description: Value must be a numeric gain value, or "max", or "agc".
 Type: error
+
+Template: dump1090-mutability/invalid-is_non_root_user
+Description: Value must be a username (without spaces) that isn't root.
+Type: error