* add function that can clear cached config files

Once the first call to `Get()`, `GetStrict()`, `GetAll()`,
or`GetAllStrict()` has been made, the contents of the config files will
be cached for all future calls to any of those functions. This can be
frustrating if the user wants to capture changes to the config file that
were made externally. This
change adds the `ClearCachedConfigs()` function to provide control over
that.

* change ClearCachedConfigs to ReloadConfigs

Clearing cached data is somewhat vague and could be more confusing to
users who are not familiar with this library. But Reloading the config
data is very straightforward. For this reason, the ability to clear the
cache has been replaced with the ability to reload.

The reload simply clears the cache and then loads the contents again.

Additionally, this contains a bug fix that ensure the loadConfigs
pointer is set to a non-null value before it is used.

* add tests for reloading ssh config files

This adds 2 tests. The first checks that ssh config data is cached and
does not update if the file changes afterward. The second checks that
the `ReloadConfigs()` function will discard the cached data and load the
current config file contents.

Credit to @virtuald for the original implementation of this fix.

* Add support for retrieving all IdentityFile directives via DefaultAll

* fix: set IdentityFile defaults to match man page

The existing default IdentityFile list was incomplete and out of order.
This updates it to match the defaults listed here:
https://man7.org/linux/man-pages/man5/ssh_config.5.html

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Co-authored-by: Dustin Spicuzza <[email protected]>

The new ignoreMatchDirective flag needed to be added in one other place.