Add extra-opts documentation

Import the --extra-opts documentation from our old site.  The --help
output of plugins refers to <http://nagiosplugins.org/extra-opts>.

1 files changed, 72 insertions, 0 deletions

diff --git a/web/content/doc/extra-opts.md b/web/content/doc/extra-opts.md
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+title: Extra-Opts
+parent: Documentation
+---
+
+# Extra-Opts
+
+Starting with the 1.4.12 release, Nagios C plugins support reading
+configuration options from a configuration file.  This needs to be enabled at
+compile-time for now (`--enable-extra-opts`) and will be enabled by default in
+the future.  Perl plugins using the Nagios::Plugin module have this support
+since version 0.16.
+
+You can easily know if a plugin supports *extra-opts* by checking the `--help`
+output for the `--extra-opts` option.  Once compiled in, the `--extra-opts`
+plugin option allows reading extra options from a config file.  The syntax for
+the command is:
+
+    --extra-opts=[section][@file]
+
+Some examples:
+
+* Read `special_opts` section of default config file:
+
+        ./check_stuff --extra-opts=special_opts
+
+* Read `special_opts` section of `/etc/myconfig.ini`:
+
+        ./check_stuff --extra-opts=special_opts@/etc/myconfig.ini
+
+* Read `check_stuff` section of `/etc/myconfig.ini`:
+
+        ./check_stuff --extra-opts=@/etc/myconfig.ini
+
+* Read `check_stuff` section of default config file and use additional
+  arguments along with the other specified arguments (*extra-opts* arguments
+  are always processed first no matter where `--extra-opts` appears on the
+  command line):
+
+        ./check_stuff --extra-opts -jk --some-other-opt
+
+The default nagios plugins file is used if no explicit filename is given.  The
+current standard locations checked are:
+
+* `/etc/nagios/plugins.ini`
+* `/usr/local/nagios/etc/plugins.ini`
+* `/usr/local/etc/nagios/plugins.ini`
+* `/etc/opt/nagios/plugins.ini`
+* `/etc/nagios-plugins.ini`
+* `/usr/local/etc/nagios-plugins.ini`
+* `/etc/opt/nagios-plugins.ini`
+
+To use a custom location, set a `NAGIOS_CONFIG_PATH` environment variable to
+the set of directories that should be checked (this is a colon-separated list
+just like `PATH`).  The first `plugins.ini` or `nagios-plugins.ini` file found
+in these directories will be used.
+
+To specify an option without parameter, you can use a key without value, but
+the equal sign must remain, for example:
+
+    allow-regex=
+
+Also note that repeated keys are allowed within sections just like you can
+repeat arguments on the command line.
+
+The basic theory is that options specified in the configuration files are
+substituted at the beginning of the command line.
+
+The initial use case for this functionality is for hiding passwords, so you do
+not have to define sensitive credentials in the Nagios configuration and these
+options won't appear in the command line.
+
+<!--% # vim:set filetype=markdown textwidth=78 joinspaces: # %-->