osx: add a target to build an OSX package

We use pkgbuild and productbuild (which seem to have superseded
PackageMaker). Those tools are shipped from OSX 10.6.6. There are only
a few examples of how to use those tools with daemons. You may also
look at Jenkins or ngircd.

See `README.md` for more information on how to invoke the build of a
package.
osx: simpler instructions in README.md
osx: use HTML files instead of RTF for welcome message

Also add a conclusion file.
configure: ensure that osx/scripts are executable
snmp: fix a typo in `agent_priv.c`