Fix potential out-of-bounds read

Fix compiler warning: loop without body

- Fixing out of bounds memory overwrite triggered with malformed /etc/hosts file.
- Improving parsing of /etc/hosts file.
- Validating requested address family.
- Ensuring that failures always return a NULL pointer.
- Adjusting header inclusions.

target as indicated end requested by Viktor Szakats in bug report #2872879.

  (http://curl.haxx.se/bug/view.cgi?id=2870221) that libcurl returned an
  incorrect return code from the internal trynextip() function which caused
  him grief. This is a regression that was introduced in 7.19.1 and I find it
  strange it hasn't hit us harder, but I won't persue into figuring out
  exactly why.

  SO_SNDBUF to CURL_WRITE_SIZE even if the SO_SNDBUF starts out larger.  The
  patch doesn't do a setsockopt if SO_SNDBUF is already greater than
  CURL_WRITE_SIZE. This should help folks who have set up their computer with
  large send buffers.

make errno available as a preprocessor macro.

  the define CURL_MAX_HTTP_HEADER which is even exposed in the public header
  file to allow for users to fairly easy rebuild libcurl with a modified
  limit. The rationale for a fixed limit is that libcurl is realloc()ing a
  buffer to be able to put a full header into it, so that it can call the
  header callback with the entire header, but that also risk getting it into
  trouble if a server by mistake or willingly sends a header that is more or
  less without an end. The limit is set to 100K.

  saving received cookies with no given path, if the path in the request had a
  query part. That is means a question mark (?) and characters on the right
  side of that. I wrote test case 1105 and fixed this problem.

  transfer.c for blocking. It is currently used only by SCP and SFTP protocols.
  This enhancement resolves an issue with 100% CPU usage during SFTP upload,
  reported by Vourhey.

  (http://curl.haxx.se/bug/view.cgi?id=2861587) identifying that libcurl used
  the OpenSSL function X509_load_crl_file() wrongly and failed if it would
  load a CRL file with more than one certificate within. This is now fixed.

added support for new SQLite cert database format: added a runtime check for version 3.12.0, and depending on the result add 'sql:' prefix to cert database directory so that newer SQLIte database format works.