Add main config include support

This works by you adding (probably) include_dir or (unlikely)
include_file into your nagios.cfg, and point it to a file where modules
can dump their config. If you use include_dir, make sure your config
files are called whatever.cfg, or naemon will proceed to ignore the
file - this lets a user dump, say, a README file into their config dir,
without naemon complaining that it's not a valid config file.

The settings in the subfile(s) are included as if the contents of the
subfile had been at the line that contains the include line, just like a
C preprocessor statement.

Perhaps unintuitively (or not, depending on what your intuition tells
you), all relative paths are relative to the main config file. While
it's not the case technically, you should pretend that naemon chdir()ed
into the directory where the main naemon.cfg file is located, and then
resolved all paths relative to the cwd.

In order for this to work with cfg_file and cfg_dir statements - which
used to be parsed by a separate nagios.cfg "parser" (because why
anything?), I've introduced two more globals. I feel very bad about
this.

Change-Id: I61165280ea0ef7a48e5bfdd8bc6019f9fa84401b
Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>