Undo old temporary change once used for testing purposes

Symbol hiding configure options renamed to the hopefully less ambiguous
--enable-symbol-hiding and --disable-symbol-hiding as well as related
macro names and some internal variables used for them.

Related configuration file preprocessor symbols named to
CARES_SYMBOL_HIDING and CARES_SYMBOL_SCOPE_EXTERN.

Fix two typos.

shows that this one is actually a modified copy of ares_parse_a_reply.c.

In order to comply with ares_parse_a_reply.c's M.I.T. license, the old
1998 M.I.T. copyright notice is now also preserved in this file the same
as it is done in other ares_parse_*.c files.

Fix identation of some license lines.

ares_parse_txt_reply() current version:

- Fixed a couple of potential double free's.

- Fixed memory leaks upon out of memory condition.

- Fixed pointer arithmetic.

- Setting ntxtreply to zero upon entry for all failure cases.

- Changed data type to size_t for variables substr_len, str_len and
  the length member of ares_txt_reply struct.

- Avoided a couple of memcpy() calls.

- Changed i data type to unsigned int to prevent compiler warnings.

- Adjusted a comment.

- Use ARES_SUCCESS literal for successfull completion.

- Added CVS Id tag.

dynamic and static c-ares libraries in debug and release flavours.

Additionally each of the three sample programs is built against
each of the four possible c-ares libraries, generating all this
a total number of 12 executables and 4 libraries.

MUST be valid and they are dereferenced further down in the function
unconditionally!

based on the 'visibility' attribute for GNUC and __global for Sun
compilers, taking also in account __declspec function decoration
for Win32 and Symbian DLL's.

Introducing configure options --enable-hidden-symbols and
--disable-hidden-symbols following libcurl's naming.

ARES_ECANCELLED error code value and missing error code description.

is only expanded and included once in the configure script.

due to autoconf 2.64 m4_require'ing it in _AS_SHELL_SANITIZE indirectly through
_AS_PATH_WALK.

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.