Check for libtinfo after libncurses

See Gentoo bug 459768 (https://bugs.gentoo.org/show_bug.cgi?id=459768)

Closes #1322 (https://github.com/fish-shell/fish-shell/pull/1322).
Changes full_escape to not generate \x escapes for non-ASCII characters.
Partially reverts 51de269 . Fixes #1225
Incorporate a modified UTF8 <-> wchar_t implementation from Alexey
Vatchenko (http://www.bsdua.org/libbsdua.html) in preparation for
eliminating our dependency on iconv
Included missing stdint.h header in utf8.cpp
Naive reimplementation of utf2wcs and wcs2utf in
env_universal_common.cpp. These use the new utf8 functions exposed in
utf8.h. This will allow us to drop the iconv dependency.
Stop linking iconv, now that we have our own utf8 conversion routines