* Revert "Revert "executor: add setuid sandbox for openbsd""
The problem is the low file descriptor limit.
This reverts commit 4093e33b13.
* executor/executor make sure the file descriptor limit is sufficient
Since the OpenBSD target does not make use of syz_execute_func yet, just drop
PROT_EXEC for now.
Supporting write and exec would require one to edit /etc/fstab during
installation.
Regression introduced in commit a4718693 ("sys/linux: add syz_execute_func").
Make as much code as possible shared between all OSes.
In particular main is now common across all OSes.
Make more code shared between executor and csource
(in particular, loop function and threaded execution logic).
Also make loop and threaded logic shared across all OSes.
Make more posix/unix code shared across OSes
(e.g. signal handling, pthread creation, etc).
Plus other changes along similar lines.
Also support test OS in executor (based on portable posix)
and add 4 arches that cover all execution modes
(fork server/no fork server, shmem/no shmem).
This change paves way for testing of executor code
and allows to preserve consistency across OSes and executor/csource.
