Remove `SYS_futex`, `SYS_write`, `SYS_close`, and `SYS_mmap` from list of default allowed syscalls for seccomp on Linux

[danbugs]

Currently, we allow SYS_futex, SYS_write, SYS_close, and SYS_mmap (

hyperlight/src/hyperlight_host/src/seccomp/guest.rs

Line 56 in b9c67fb

(libc::SYS_futex, vec![]),

) by default beause they are needed by some writer functions we have. Writer functions are not registered like normal host functions. Instead, they are passed in as a parameter to UninitializedSandbox::new (

hyperlight/src/hyperlight_host/src/sandbox/uninitialized.rs

Line 125 in b9c67fb

host_print_writer: Option<&dyn HostFunction1<String, i32>>,

) and, so, allowing extra syscalls to it is a bit cumbersome. We should consider refactoring UninitializedSandbox::new to leverage a builder pattern.