etc/nsca-ng.cfg: Add a default "authorize" section

Add an "authorize" section which lets the server start out of the box
(instead of bailing out because the user didn't specify an "authorize"
section).  The new configuration still doesn't permit clients to submit
anything to Nagios (it just allows authenticated clients to talk to the
server), so the client will spit out a "not authorized" message until
the user edits the nsca-ng.cfg(5) file.
etc/nsca-ng.cfg: Give better examples

Improve the clarity of the authorization section examples a bit.
configure.ac: Set AC_CONFIG_MACRO_DIR()

The Autoconf manual suggests to specify the AC_CONFIG_MACRO_DIR(), as it
might be used "by future versions of commands like autoreconf that trace
macro calls".
Mark Markdown files as such for Vim

Specify "filetype=markdown" in the Vim modeline of Markdown files.
PROTOCOL: Improve wording a bit

Slightly improve the wording of the description of the PUSH request.
Make Clang happy

Fix two minor compiler warnings.
COPYING: Slightly improve wording

Improve the wording in the COPYING file a bit.
contrib/debug_server: Fix send_nsca.cfg(5) parsing

Quote the argument to the "eval" call that parses the configuration
file.
contrib/downtime: Fix our timestamps

The timestamps must be submitted as the number of seconds since the
Epoch.
contrib/invoke_check: Fix our send_nsca(8) call

The invoke_check script doesn't feed send_nsca(8) with "raw" commands,
so specifying the "-C" option was incorrect.