* .travis.yml: Install libidn2-dev instead libidn11-dev.
* bootstrap.conf: Add modules libunistring-optional, unistr/base,
unicase/tolower.
* configure.ac: Check for libidn2.
* src/Makefile.am: Add $(LTLIBUNISTRING) to LDADD.
* tests/Makefile.am: Set LDADD similar to LDADD in src/Makefile.am
* src/connect.c: Use libidn2 code instead of libidn.
* src/host.c: Likewise.
* src/iri.c: Likewise.
* src/iri.h: Likewise.
* src/options.h: Likewise.
* src/url.c: Likewise.
* src/url.h: Likewise.
* src/log.c: Fix C99 comment.
IDN2003 should not be used any more due to security concerns.
We use libunistring (resp. the unicode code from gnulib) for
lowercasing UTF-8 before we give data to libidn2.
TR#46 is missing, no support in libidn2 nor in libunistring.
* src/iri.c (remote_to_utf8): Do not qualify with const the output pointer.
(do_conversion): Use the provided input parameter as const.
(idn_encode): casts to remote_to_utf8 parameters are no longer needed.
* src/iri.h: Adjusted remote_to_utf8 prototype.
* src/url.c: It is no longer necessary to cast new_url to const char.
xfree() might crash on libidn memory on Windows.
From 'man idn_free':
"Under Windows, different parts of the same application may use different
heap memory, and then it is important to deallocate memory allocated within
the same module that allocated it. This function makes that possible."