uhttpd: update to latest Git head

db86175 lua: honour size argument in recv() function
d3b9560 utils: add uh_htmlescape() helper
8109b95 file: escape strings in HTML output
393b59e proc: expose HTTP Origin header in process environment
796d42b client: flush buffered SSL output when tearing down client ustream

Signed-off-by: Jo-Philipp Wich <jo@mein.io>