sample-config: Generate at build time

The old code generated sample configs full of "${prefix}", which is not
a variable naemon knows how to parse.

While we're at it, install the sample config as part of "make install",
but don't install it if the target config directory already exists.

Signed-off-by: Robin Sonefors <ozamosi@flukkost.nu>
Makefile.am: Install everything by default

This should ensure that after "make install", you can start naemon or
install the init script straight away, without needing to create
files/directories that were required but weren't created.

Signed-off-by: Robin Sonefors <ozamosi@flukkost.nu>