plugin-loader: Optionally use load option in each plugin section to load plugins

This now works because all plugins use the same config namespace.

If <ns>.load_modular is true, the list of plugins to load is determined
via the value of the <ns>.plugins.<name>.load options.

Using includes the following is possible:

charon {
  load_modular = yes
  plugins {
    include strongswan.d/charon/*.conf
  }
}

charon-cmd {
  load_modular = yes
  plugins {
    include strongswan.d/charon-cmd/*.conf
  }
}

Where each .conf file would contain something like:

<name> {
  load = yes
  <option> = <value>
}

To increase the priority of individual plugins load = <priority> can be
used (the default is 1).  For instance, to use openssl instead of the
built-in crypto plugins set in strongswan.d/charon/openssl.conf:

openssl {
  load = 10
}

If two plugins have the same priority their order in the default plugin
list is preserved.  Plugins not found in that list are ordered
alphabetically before other plugins with the same priority.

man/strongswan.conf.5.in

@@ -290,6 +290,15 @@ Threshold in number of allocations for leaks to be reported (0 to report all)
 .BR charon.load
 Plugins to load in the IKEv2 daemon charon
 .TP
+.BR charon.load_modular " [no]"
+If enabled, the list of plugins to load is determined via the value of the
+charon.plugins.<name>.load options.  In addition to a simple boolean flag that
+option may take an integer value indicating the priority of a plugin, which
+would influence the order of a plugin in the plugin list (the default is 1).
+If two plugins have the same priority their order in the default plugin list
+is preserved. Enabled plugins not found in that list are ordered alphabetically
+before other plugins with the same priority.
+.TP
 .BR charon.max_packet " [10000]"
 Maximum packet size accepted by charon
 .TP