This function is called during very early startup and which can result
in a crash on FreeBSD. The sigaction() function in libc is indirected
via a table so that it can be interposed by the threading library
rather than calling the syscall directly. In the crash I was observing
this table had not yet been relocated, so we ended up jumping to an
invalid address. To avoid this problem we can call __sys_sigaction,
which calls the syscall directly and in FreeBSD 15 is part of libsys
rather than libc, so does not depend on libc being fully initialized.
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