Add platform-independent fontspec_new_default() function.

Constructing a FontSpec in platform-independent code is awkward,
because you can't call fontspec_new() outside the platform subdirs
(since its prototype varies per platform). But sometimes you just need
_some_ valid FontSpec, e.g. to put in a Conf that will be used in some
place where you don't actually care about font settings, such as a
purely CLI program.

Both Unix and Windows _have_ an idiom for this, but they're different,
because their FontSpec constructors have different prototypes. The
existing CLI tools have always had per-platform main source files, so
they just use the locally appropriate method of constructing a boring
don't-care FontSpec.

But if you want a _platform-independent_ main source file, such as you
might find in a test program, then that's rather awkward. Better to
have a platform-independent API for making a default FontSpec.

test/fuzzterm.c

@@ -201,7 +201,7 @@ int platform_default_i(const char *name, int def)
 
 FontSpec *platform_default_fontspec(const char *name)
 {
-    return fontspec_new("");
+    return fontspec_new_default();
 }
 
 Filename *platform_default_filename(const char *name)