portable: add siphash implementation Siphash is being used in place of murmurhash for object-id set as this is faster. However, this isn't really seen much in the non-BSD userspace, so provide an implementation for it portably, if one isn't found.

portable: improve macos compat with bison/openssl Some MacOS systems might not have GNU Bison installed, or have openssl installed. In such instances the host versions of those can be used, with a few tweaks around the edges to make them work. This commit addresses these by: * Relaxing a HAVE_CRYPTO check for __APPLE__, since the inclusion of the machine-specific headers isn't dependent on openssl being installed; * If the non-GNU version of bison on MacOS is being used, define any missing YY* variables, and add a compiler #warning so we log something. This also updates the CI scripts to default to the system versions of bison and removes openssl as a core dependency -- it's more likely openssl is going to be installed, so removing it here will mean we can catch the case for just the MacOS-specific failures, and the inclusion of openssl on other OSes should cover the rest. Noticed by Evan Silberman

portable: add support for DragonFlyBSD This adds support for DragonFlyBSD.

portable: include support for MacOS This adds support for MacOS (Monteray onward, although should work on older MacOSes). The BSD wrapper for poll/ppoll has been lifted from the work the folks over at OpenSSH have done -- thanks!

portable: make UUID includes generic Move the detection of UUID header files to be included portably. This will have no functional effect until MacOS detection is put in place.

portable: add support for landlock landlock is a new set of linux APIs that is conceptually similar to unveil(2): the idea is to restrict what a process can do on a specified part of the filesystem. There are some differences in the behaviour: the major one being that the landlock ruleset is inherited across execve(2). This just restricts the libexec helpers by completely revoking ANY filesystem access; after all they are the biggest attack surface. got send/fetch/clone *may* end up spawning ssh(1), so at the moment is not possible to landlock the main process. From Omar Polo.

portable: add compat wrappers for timespec*() Although there are definitions for timespec*() functions in libbsd, these were added fairly recently, which means not all non-BSD systems will have them. In such cases, provide a portable wrapper.

portable: add FreeBSD support This adds the capability to compile got-portable on FreeBSD.

portable: initial Linux compilation This commit modifies the GoT main branch to be able to compile it under linux.