fmt (some missing space -> tab conversion)

emit a dummy NUL byte on empty files an empty initializer, such as uint8_t foo[] = { }; raises a warning: "use of GNU empty initializer extension" using -pedantic. This adds a dummy NUL byte that's not accounted in the len counter. So, now it produces: uint8_t foo[] = { 0x0 }; size_t foo_len = 0;

don't add trailing NUL byte plus some doc improvements and a missing return

added missing include thanks Ben for noticing it!

move built-in pages to pages/*.gmi