net/dante: restore read/send-side pipe system check during configure

It was disabled (patched out) in July 2023 based on the pkg-fallout@
emails, nearly seven months after the last regression which could be
causing it had been fixed. Evidently, the cluster was running buggy
kernel for its port-building jails long enough to confuse people the
problem was still somehow exhibiting itself.

While here, assume maintainership, fix some DRY bugs in the Makefile,
and install the manpages under $PREFIX/share/man per new world order.

PR: 272755
Fixes: 2b81a5f3f73c