Ref: https://httpwg.org/specs/rfc7230.html#header.fields

  field-value    = *( field-content / obs-fold )
  field-content  = field-vchar [ 1*( SP / HTAB ) field-vchar ]
  field-vchar    = VCHAR / obs-text

- we do not accept the obsolete line folding (obs-fold)
- VCHAR is 0x20 to 0x7e
- obs-text is 0x80 to 0xff

So we end up with !VCT_CTL || HTAB (LF)

This change would be the basis for new use cases, like

- a type method .valid_header
- a feature flag +validate_headers which would call VRT_ValidHdr
  for all SetHdr calls