Introduce new Sphinx role iscman for ISC manual pages

The new directive and role "iscman" allow to tag & reference man pages in
our source tree. Essentially it is just namespacing for ISC man pages,
but it comes with couple benefits.

Differences from .. _man_program label we formerly used:
- Does not expand :ref:`man_program` into full text of the page header.
- Generates index entry with category "manual page".
- Rendering style is closer to ubiquitous to the one produced
  by ``named`` syntax.

Differences from Sphinx built-in :manpage: role:
- Supports all builders with support for cross-references.
- Generates internal links (unlike :manpage: which generates external
  URLs).
- Checks that target exists withing our source tree.

(cherry picked from commit 7e7a946d44)

bin/plugins/filter-a.rst

@@ -11,6 +11,7 @@
 
 .. highlight: console
 
+.. iscman:: filter-a
 .. _man_filter-a:
 
 filter-a.so - filter A in DNS responses when AAAA is present

bin/plugins/filter-aaaa.rst

@@ -11,6 +11,7 @@
 
 .. highlight: console
 
+.. iscman:: filter-aaaa
 .. _man_filter-aaaa:
 
 filter-aaaa.so - filter AAAA in DNS responses when A is present