add option --s-rd-sz (socket read size):

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

docs/devnotes.md

@@ -164,6 +164,7 @@ authenticate using header `Cookie: cppwd=foo` or url param `&pw=foo`
 | PUT | `?xz` | (binary data) | compress with xz and write into file at URL |
 | mPOST | | `f=FILE` | upload `FILE` into the folder at URL |
 | mPOST | `?j` | `f=FILE` | ...and reply with json |
+| mPOST | `?replace` | `f=FILE` | ...and overwrite existing files |
 | mPOST | | `act=mkdir`, `name=foo` | create directory `foo` at URL |
 | POST | `?delete` | | delete URL recursively |
 | jPOST | `?delete` | `["/foo","/bar"]` | delete `/foo` and `/bar` recursively |