[ ] remove typedef for FeedItem, FeedTag etc.
[ ] fix timezone in slashdot.xml
    [ ] string_append: make sure some fields can only be set once (first field counts?).
    [ ] test new parsetime and gettimetz code.

[ ] grep -H -n *.c 'TODO:'
[ ] sfeed_frame: snprintf check truncated, <= 0 and >= size.
[ ] sfeed.c: document ctx.iscontent and ctx.iscontenttag.
[ ] use openbsd sys/queue.h for util.c linked-list and sfeed_frames.c
[ ] rewrite xml.{c,h}
[ ] xml: check read errors:
	ferror, return code of fread, fwrite, fclose...
[ ] 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
[ ] feeds file: empty tab (encoding can't be detected?)
[ ] optimize
	[ ] gettag() lookup.
	[ ] namedentitytostr lookup.
	[ ] xml parser, etc.
[?] add sfeed_atom ? to aggregate feeds?