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.

unix/pageant.c

@@ -172,7 +172,7 @@ void random_destroy_seed(void) {}
 char *platform_default_s(const char *name) { return NULL; }
 bool platform_default_b(const char *name, bool def) { return def; }
 int platform_default_i(const char *name, int def) { return def; }
-FontSpec *platform_default_fontspec(const char *name) { return fontspec_new(""); }
+FontSpec *platform_default_fontspec(const char *name) { return fontspec_new_default(); }
 Filename *platform_default_filename(const char *name) { return filename_from_str(""); }
 char *x_get_default(const char *key) { return NULL; }