Improve and extend tests on man pages support.

Improve and extend tests on man pages support.

* tests/man.test: Enable `errexit' shell flag, and related changes.
Make grepping of generated Makefile.in slighty stricter.
* tests/man3.test:  Add trailing `:' command.
* tests/man5.test: Prefer cat + here-doc over echo to append to
configure.in.
* tests/man2.test: Likewise, and add trailing `:' command.
* tests/man4.test: More thorough and consistent checking of make
error messages.  Place fake `help2man' program in a new `bin'
directory rather than in `.'.  Move the checks using the real
`help2man' program to ...
* tests/man6.test: ... this new test, and extend them.  This test
passes with GNU make and Solaris make, still fails with BSD make.
* tests/man7.test: New test, extracted from old man4.test, which
checks for a bug in maintainer-clean w.r.t. generated manpages.
* tests/man8.test: New test, extracted from old man4.test, which
checks for a bug in distcheck w.r.t. generated manpages.  Passes
with GNU make and Solaris make, still fails with BSD make.