Introduce html.h

All html-functions can be quite easily separated from the rest of cgit, so
lets do it; the only issue was html_filemode which uses some git-defined
macros so the function is moved into ui-shared.c::cgit_print_filemode().

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.c

@@ -291,7 +291,6 @@ int main(int argc, const char **argv)
 	const char *cgit_config_env = getenv("CGIT_CONFIG");
 
 	cgit_prepare_context(&ctx);
-	htmlfd = STDOUT_FILENO;
 	item.st.st_mtime = time(NULL);
 	cgit_repolist.length = 0;
 	cgit_repolist.count = 0;