package kit: Add pre-uninstall scripts feature.

Just like post-install scripts which run shortly after a package is
installed, pre-uninstall scripts are run just before a package is
removed. Implements enhancement #13427

* Fix script exit code handling vs script launch errors.
* Bump package and repo file version numbers due to new attribute,
unfortunately makes new .hpkg files not backwards compatible.
* Add pre-uninstall functionality, mostly cloning the post-install
except in a few places.
* Discover that _RunQueuedScripts() is never called, a future TODO:?
* Update package documentation for pre-uninstall scripts, and use of
the boot/post-install directory.

Change-Id: I45596255ce74bc102f6e5b606cbf83e4e4347a17
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1504
Reviewed-by: Alexander G. M. Smith <agmsmith@ncf.ca>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

sparc: add defines and minimum set of required files

Gets the stage0 bootstrap to run.
Imlementation is probably nonsense at this point.

Change-Id: I10876efbb54314b864c0ad951152757cdb2fd366
Reviewed-on: https://review.haiku-os.org/c/1061
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

docs/develop/packages: convert wiki to rst

Cleaner syntax compared to MediaWiki, more features than Markdown, while
still looks good on plain text.