Update imsg from libutils via OpenBSD. Needed for some imsg->fd handling.

Sync latest changes of imsg from OpenBSD, as some of its newer fd handling is required.

Rather than assume all dependencies are required for all programs, split them out. This will make packaging easier, as well as splitting the code to use subprojects. Note that due to the use of config.h semantics, in most cases the got_compat.h header file is now at the top of the .c file it is included in, so that it can handle the system header inclusion properly.

There is only one user which calls xrecallocarray: ibuf_realloc(). We already provide recallocarray() in the compat layer so ibuf_realloc() can simply call recallocarray() directly, as it does on OpenBSD. OK thomas.adam

This commit modifies the GoT main branch to be able to compile it under linux.