plugin: sort topological candidates by specified order.

We previously registered hooks up in who-replies-to-getmanifest-first
order, but then if any had dependencies it would scatter that order.

This allows users to manually set dependencies developers have
forgotten by specifying the plugins manually in their configuration or
cmdline.  This was an excellent consideration by @mschmook.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightningd-config.5

@@ -519,14 +519,17 @@ additional paths too:
 
  \fBplugin\fR=\fIPATH\fR
 Specify a plugin to run as part of c-lightning\. This can be specified
-multiple times to add multiple plugins\.
+multiple times to add multiple plugins\.  Note that unless plugins themselves
+specify ordering requirements for being called on various hooks, plugins will
+be ordered by commandline, then config file\.
 
 
  \fBplugin-dir\fR=\fIDIRECTORY\fR
 Specify a directory to look for plugins; all executable files not
 containing punctuation (other than \fI\.\fR, \fI-\fR or \fI_) in 'DIRECTORY\fR are
 loaded\. \fIDIRECTORY\fR must exist; this can be specified multiple times to
-add multiple directories\.
+add multiple directories\.  The ordering of plugins within a directory
+is currently unspecified\.
 
 
  \fBclear-plugins\fR
@@ -581,4 +584,4 @@ Main web site: \fIhttps://github.com/ElementsProject/lightning\fR
 Note: the modules in the ccan/ directory have their own licenses, but
 the rest of the code is covered by the BSD-style MIT license\.
 
-\" SHA256STAMP:6b275a3227db6565ad3c5369b95d3eefe9472864b8ed9e6c5c768981cdf23b8f
+\" SHA256STAMP:c927bd3afb61288bb67d941d113cdeefe1363b0c7a28936ef30d43af3ce66098