This function casts its argument to int32 before passing it to the
system call, so that big-endian CPUs read the lower rather than the
upper 32 bits of the pid.
This also gets rid of the last import of "unsafe" in the Unix build.
I changed syscall to x/sys/unix while I was at it, to remove one more
import line. The constants and types there are aliases for their syscall
counterparts.
The restic security model includes full trust of the local machine, so
this should not fix any actual security problems, but it's better to be
safe than sorry.
Fixes #2192.
