config: improve validation of color values, default alpha to 0xff

Reject color values that aren't in either RGB, or ARGB format. That
is, color values that aren't hexadecimal numbers with either 6 or 8
digits.

Also, if a color value is allowed to have an alpha component, and the
user left it out, default to 0xff (opaque) rather than 0x00 (fully
transparent).

Closes #1526

tests/test-config.c

@@ -426,6 +426,10 @@ test_color(struct context *ctx, bool (*parse_fun)(struct context *ctx),
         {"ffffff", 0xffffff},
         {"ffffffff", 0xffffffff, !alpha_allowed},
         {"aabbccdd", 0xaabbccdd, !alpha_allowed},
+        {"00", 0, true},
+        {"0000", 0, true},
+        {"00000", 0, true},
+        {"000000000", 0, true},
         {"unittest-invalid-color", 0, true},
     };