New locations for config files and support binaries, and smarter default for install...

New locations for config files and support binaries, and smarter default for install root directory.

The default locations for config files and back-end executables has
changed.

Config files have moved from ${sysconfdir} to ${sysconfdir}/nmh, and
the back-end programs from ${libdir} to ${libexecdir}/nmh.

The only user-visible change is the addition of a new mhparam(1)
component 'libexecdir', which replaces the previous 'libdir'.  User
scripts invoking 'mhparam libdir' will need an update.  'libdir'
has been kept around for the time being, to give people a chance
to update their scripts.  It returns the same value as 'libexecdir'.
'libdir' has been marked deprecated in the mhparam(1) manpage and
the release notes; it will be removed in a couple of releases.

In conjunction with this change there was an opportunity to do a
bit of Makefile cleanup.  The old 'auxexec' construct has been
replaced with a new scheme that leverages automake's templating.
The auxexec_* references have been replaced with nmhlibexec_* forms,
and a corresponding nmhlibexecdir variable defined as ${libexecdir}/nmh.

Likewise, etcdir has been replaced with nmhetcdir.

These name changes have percolated downstream to some extent, primarily
in the man pages and test scripts.

Finally, configure has been made a bit smarter about setting the
default ${prefix}.  If it finds an existing nmh installation, it
will set the default ${prefix} to match, falling back to /usr/local/nmh.
configure looks for an existing installation by searching $PATH for
a viable mhparam binary.