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; }
 

unix/plink.c

@@ -64,7 +64,7 @@ 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)

unix/psusan.c

@@ -78,7 +78,7 @@ 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)

unix/sftp.c

@@ -52,7 +52,7 @@ 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)

unix/uppity.c

@@ -80,7 +80,7 @@ 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)

unix/utils/fontspec.c

@@ -13,6 +13,11 @@ FontSpec *fontspec_new(const char *name)
     return f;
 }
 
+FontSpec *fontspec_new_default(void)
+{
+    return fontspec_new("");
+}
+
 FontSpec *fontspec_copy(const FontSpec *f)
 {
     return fontspec_new(f->name);

unix/window.c

@@ -304,7 +304,7 @@ FontSpec *platform_default_fontspec(const char *name)
     if (!strcmp(name, "Font"))
         return fontspec_new(DEFAULT_GTK_FONT);
     else
-        return fontspec_new("");
+        return fontspec_new_default();
 }
 
 Filename *platform_default_filename(const char *name)