libsel4muslcsys: Implement sys_write wrapper

This wraps sys_writev the same way that sys_read is implemented.

pragma once and for all

Fix licenses

libsel4muslcsys: Add support for openat syscall

libsel4muslcsys: Implement mmap syscall

Changes the current mmap2 implementation to be an mmap implementation,
and redirects the mmap2 syscall to mmap

libsel4muslcsys: create dummy implementation of sys_madvise()

libsel4muslcsys: Add some missing thread routines

A C library may call set_thread_area on startup and tkill on shutdown,
so we need to implement stubs of these