doc: build man pages at build time; introduce HAVE_SPHINX, HAVE_RST2MAN

This helps avoid build artifacts (namely, nroff and gzipped-nroff man
pages) owned by root.

The variables allow choosing which generator to use for the man page.
These will be hooked to configure in a following commit.
doc: build man pages into hierarchy, fix help test.

It turns out there was a reason the old man pages were stored in a man
compatible hierarchy, namely so that we could run man on them before
installing.

Hardcode doc build location into test suite.  This isn't ideal, but
let's unbreak the test suite for now.
doc: configure detection of sphinx and rst2man

Because sphinx-build does not provide a convenient way of listing
which builders exist, and some people actually have pre 1.0 sphinx, we
try loading a relevant python module.

Currently the assumption is that no python in path -> no sphinx-build
in path.
doc: cosmetic fix for prerst2man.py

Fix a particular egregious combination of format and string
concatenation.