This is just in case there is some rogue platform that uses a
nonstandard value for SEEK_*, and does not define that macro in
unistd.h. I think that's unlikely, but it's conceivable.
This patch:
- introduces an fdio module for low-level fd functions that don't
need to log.
- moves the responsibility for opening files outside of torlog.c,
so it won't need to call tor_open_cloexec.
