Bump libweb

The following commits introduced performance improvements and bugfixes:

Author: Xavier Del Campo Romero <xavi.dcr@tutanota.com>
Date:   Sat Jan 20 01:09:18 2024 +0100

    server.c: Fix wrong priority for do_exit

    Under some specific circumstances, poll(2) would return a positive
    integer, but do_exit might had been previously set. This caused libweb
    to ignore SIGTERM, with the potential risk for an endless loop.

Author: Xavier Del Campo Romero <xavi.dcr@tutanota.com>
Date:   Sat Jan 20 01:05:05 2024 +0100

    http.c: Solve performance issues on POST uploads

    Profiling showed that reading multipart/form POST uploads byte-by-byte
    was too slow and typically led to maximum CPU usage. Therefore, the
    older approach (as done up to commit 7efc2b3a) was more efficient, even
    if the resulting code was a bit uglier.

libweb

@@ -1 +1 @@
-Subproject commit b0accd099fa8c5110d4c3c68830ad6fd810ca3ec
+Subproject commit 91ff743df9fcee048cb44dba0c7047df04454dd2