[ ] use openbsd sys/queue.h for util.c linked-list and sfeed_frames.c

[ ] check read errors:
	ferror, return code of fread, fwrite, fclose...

[x] handle invalid value in strtoul.
[x] compile with -Wextra and use (void) to ignore unused parameters?
[x] use uint32_t for codepointtoutf8?

[ ] separate codepointtoutf8 and entity handling from sfeed.c into xml.c?

[ ] test again on:
	- Linux (glibc+gcc, musl-gcc, clang, tcc).
	- OpenBSD
	- Windows (cygwin gcc, mingw).
[ ] test shellscripts with ubase/sbase tools.
	[ ] sfeed_update
	[ ] sfeed_opml_export
[ ] remove sfeed_stats ?
[ ] dogfood sfeed_frames, html content.
	[ ] in links browsers: items are centered in the table for some reason.
[ ] feeds file: empty tab (encoding can't be detected?)
[ ] optimize
	[ ] gettag() lookup.
	[ ] namedentitytostr lookup.
	[ ] xml parser, etc.
[ ] cleanup code.