build: generate doc/*.1 files; include help2man

build: generate doc/*.1 files; include help2man

This change is required to avoid making a distributed file depend
on a generated (non-distributed) one.  The preceding change
introduced one such dependency, with the distributed doc/*.1
depending on the generated aclocal and automake files.  Here,
we avoid the problem by generating the doc/*.1 files rather than
distributing them.

* doc/help2man: New file, version 1.37.1.
* Makefile.am (EXTRA_DIST): Add doc/help2man.
(man1_MANS): Rename from $(dist_man1_MANS).  Remove $(srcdir) prefix.
(CLEANFILES): Add these files here, rather than to
$(MAINTAINERCLEANFILES), since we are no longer distributing them.
(update_mans): Use doc/help2man, not $(HELP2MAN).
* configure.ac: Don't test for help2man, now that we bundle it.