Update HTTP implementation to support field values larger than HTTP_MAX_VALUE

Update HTTP implementation to support field values larger than HTTP_MAX_VALUE
bytes (Issue #5216)

cups/http-private.h:
- Move all char * shadow variables to new fields array, rename old fields array
  to _fields (binary compatibility).

cups/http.c:
- Add new http_add_field that can handle appending values and allocating or
  clearing strings as needed.
- Update httpSetField to use http_add_field.
- Update _httpUpdate to use http_add_field instead of httpSetField.

cups/request.c:
- Fix checks for header fields.

cups/tls-*.c:
- Fix checks for Host header field.