Add a "conservative" build option

[woodruffw]

Right now, each syscall specification is defined with pretty much every possible error. This means that each syscall can fault with a wide variety of errors, but also that any given error might not make sense given the system's state and/or parameters passed to the syscall.

For example, sendto(2) specifies EAGAIN and EWOULDBLOCK as potential errors, even when the underlying socket is blocking. It's unlikely that a target application would filter error codes and notice a nonsensical ones like the above, but it wouldn't hurt to have a "conservative" build mode that only generates faulty syscalls for a sensible subset of all possible errors.

So, a spec like this:

proto: int fd, void __user *buff, size_t len, unsigned int flags, struct sockaddr __user *addr, int addr_len
parms: fd, buff, len, flags, addr, addr_len
errors:
  - EACCES
  - EAGAIN
  - EWOULDBLOCK
  - EALREADY
  - EBADF
  - ECONNRESET
  - EDESTADDRREQ
  - EFAULT
  - EINTR
  - EINVAL
  - EISCONN
  - EMSGSIZE
  - ENOBUFS
  - ENOMEM
  - ENOTCONN
  - ENOTSOCK
  - EOPNOTSUPP
  - EPIPE

would probably need to add a field like this:

basic_errors:
  - EBADF
  - ECONNRESET
  - EFAULT
# etc

To avoid duplication, we could just remove the basic_errors from the main errors list and combine the two during codegen (when not building in conservative mode).