curl: support parallel transfers

This is done by making sure each individual transfer is first added to a
linked list as then they can be performed serially, or at will, in
parallel.

Closes #3804

docs/cmdline-opts/Makefile.inc

@@ -100,7 +100,10 @@ DPAGES =					\
   noproxy.d					\
   ntlm.d ntlm-wb.d				\
   oauth2-bearer.d				\
-  output.d pass.d				\
+  output.d                                      \
+  pass.d                			\
+  parallel.d                                    \
+  parallel-max.d                                \
   path-as-is.d					\
   pinnedpubkey.d				\
   post301.d					\

docs/cmdline-opts/parallel-max.d

@@ -0,0 +1,9 @@
+Long: parallel-max
+Help: Maximum concurrency for parallel transfers
+Added: 7.66.0
+See-also: parallel
+---
+When asked to do parallel transfers, using --parallel, this option controls
+the maximum amount of transfers to do simultaneously.
+
+The default is 50.

docs/cmdline-opts/parallel.d

@@ -0,0 +1,7 @@
+Short: Z
+Long: parallel
+Help: Perform transfers in parallel
+Added: 7.66.0
+---
+Makes curl perform its transfers in parallel as compared to the regular serial
+manner.