counterpart of `--s-wr-sz` which existed already
the default (256 KiB) appears optimal in the most popular scenario
(linux host with storage on local physical disk, usually NVMe)
was previously 32 KiB, so large uploads should now use 17% less CPU
also adds sanchecks for values of `--iobuf`, `--s-rd-sz`, `--s-wr-sz`
also adds file-overwrite feature for multipart posts
some reverse-proxies expect plaintext replies, and
we don't have a brotli decompressor to satisfy this
additionally, because brotli is https-gated (thx google),
it was already an impractical mess anyways
the sfx is now 7 KiB larger
primarily to support uploading from Igloo IRC but also generally useful
(not actually tested with Igloo IRC yet because it's a paid feature
so just gonna wait for spiky to wake up and tell me it didn't work)
* error-message which explains how to run on py2 / older py3
when trying to run from source
* check compatibility between jinja2 and cpython on startup
* verify that webdeps are present on startup
* verify that webdeps are present when building sfx
* make-sfx.sh grabs the strip-hints dependency
mainly motivated by u2cli's folder syncing in turbo mode
which would un-turbo on most dupes due to wrong lastmod
disabled by default for regular http listings
(to avoid confusion in most regular usecases),
enable per-request with urlparam lt
enabled by default for single-level webdav listings
(because rclone hits the same issue as u2cli),
can be disabled with arg --dav-rt or volflag davrt
impossible to enable for recursive webdav listings