[cmake] Clean up curses vs ncurses includes

There were several issues with the way that the include tests for curses.h
were being done that were ultimately causing fish to use the headers from
ncurses but link against curses on platforms that provide an actual
libcurses.so that isn't just a symlink to libncurses.so

In particular, the old code was first testing for curses's cureses.h and then
falling back to libncurses's implementation of the same - but that logic was
reversed when it came to including term.h, in which case it was testing for
the ncurses term.h and falling back to the curses.h header. Long story short,
while cmake will link against libcurses.so if both libcurses.so and
libncurses.so are present (unless CURSES_NEED_NCURSES evaluates to TRUE, but
that makes ncurses a hard requirement), but we were brining in some of the
defines from the ncurses headers, causing SIGSEGV panics when fish ultimately
tried to access variables that weren't exported or were mapped to undefined
areas of memory in the other library.

Additionally it is an error to include termios.h prior to including the plain
Jane curses.h (not ncurses/curses.h), causing errors about unimplemented types
SGTTY/chtype. So far as I can tell, both curses.h and ncurses/curses.h pull in
termios.h themselves so it shouldn't even be necessary to manually include it,
but I have just moved its #include below that of curses.h

src/screen.cpp

@@ -15,12 +15,12 @@
 #include <unistd.h>
 #include <wchar.h>
 
-#if HAVE_NCURSES_H
+#if HAVE_CURSES_H
+#include <curses.h>
+#elif HAVE_NCURSES_H
 #include <ncurses.h>
 #elif HAVE_NCURSES_CURSES_H
 #include <ncurses/curses.h>
-#else
-#include <curses.h>
 #endif
 #if HAVE_TERM_H
 #include <term.h>