curl: add --rate to set max request rate per time unit

--rate "12/m" - for 12 per minute or
--rate "5/h" - for 5 per hour

Removed from TODO

Closes #8671

docs/TODO

@@ -149,7 +149,6 @@
  18.4 --proxycommand
  18.5 UTF-8 filenames in Content-Disposition
  18.6 Option to make -Z merge lined based outputs on stdout
- 18.7 at least N milliseconds between requests
  18.8 Consider convenience options for JSON and XML?
  18.9 Choose the name of file in braces for complex URLs
  18.10 improve how curl works in a windows console window
@@ -999,17 +998,6 @@
 
  https://github.com/curl/curl/issues/5175
 
-18.7 at least N milliseconds between requests
-
- Allow curl command lines issue a lot of request against services that limit
- users to no more than N requests/second or similar. Could be implemented with
- an option asking that at least a certain time has elapsed since the previous
- request before the next one will be performed. Example:
-
-    $ curl "https://example.com/api?input=[1-1000]" -d yadayada --after 500
-
- See https://github.com/curl/curl/issues/3920
-
 18.8 Consider convenience options for JSON and XML?
 
  Could we add `--xml` or `--json` to add headers needed to call rest API:

docs/cmdline-opts/Makefile.inc

@@ -198,6 +198,7 @@ DPAGES = \
   quote.d \
   random-file.d \
   range.d \
+  rate.d \
   raw.d \
   referer.d \
   remote-header-name.d \

docs/cmdline-opts/rate.d

@@ -0,0 +1,33 @@
+Long: rate
+Arg: <max request rate>
+Help: Request rate for serial transfers
+Category: connection
+Example: --rate 2/s $URL
+Example: --rate 3/h $URL
+Example: --rate 14/m $URL
+Added: 7.84.0
+See-also: limit-rate retry-delay
+---
+Specify the maximum transfer frequency you allow curl to use - in number of
+transfer starts per time unit (sometimes called request rate). Without this
+option, curl will start the next transfer as fast as possible.
+
+If given several URLs and a transfer completes faster than the allowed rate,
+curl will wait until the next transfer is started to maintain the requested
+rate. This option has no effect when --parallel is used.
+
+The request rate is provided as "N/U" where N is an integer number and U is a
+time unit. Supported units are 's' (second), 'm' (minute), 'h' (hour) and 'd'
+/(day, as in a 24 hour unit). The default time unit, if no "/U" is provided,
+is number of transfers per hour.
+
+If curl is told to allow 10 requests per minute, it will not start the next
+request until 6 seconds have elapsed since the previous transfer was started.
+
+This function uses millisecond resolution. If the allowed frequency is set
+more than 1000 per second, it will instead run unrestricted.
+
+When retrying transfers, enabled with --retry, the separate retry delay logic
+is used and not this setting.
+
+If this option is used several times, the last one will be used.

docs/options-in-versions

@@ -186,6 +186,7 @@
 --quote (-Q)                         5.3
 --random-file                        7.7
 --range (-r)                         4.0
+--rate                               7.84.0
 --raw                                7.16.2
 --referer (-e)                       4.0
 --remote-header-name (-J)            7.20.0