libsel4muslcsys: Implement sys_write wrapper

This wraps sys_writev the same way that sys_read is implemented.

Trivial: Style fix

make the style checker happy

seL4_libs: remove autoconf.h from seL4_libs

this commit removes autoconf.h generation from each lib and
append '#include <<lib_name>/gen_config.h>' after each
'#include <autoconf.h>' since autoconf.h is only the for the
kernel config now. This is a temporarily solution, since we
currently don't have a way to tell which header files each file
is trying to include when it includes 'autoconf.h'

libsel4muslcsys: transition to new libcpio API

Add sel4muslcsys_register_stdio_write_fn

This function is for overriding what happens when sys_writev is called
on stdout or stderr. The default stdio_write function is
sys_platform_write which calls __arch_write defined in libplatsupport
and assumes a rootserver environment.
sel4muslcsys_register_stdio_write_fn should be used in non roottask
environments to specify a custom write function as required.

Provide __arch_write

__arch_write is an alternate symbol to __arch_putchar which will
be called from sys_writev which is ultimately called by printf.

It can be used to send whole buffers at a timer, rather than one char at
a time.

If __arch_write is not provided, the default simply calls
__arch_putchar.

Fix all the whitespace

- remove trailing whitespace
- remove duplicate blank lines
- remove blank lines at end of files

Fix licenses

libsel4muslcsys: Provide run time CPIO file system support

This changes the existing build time config support for a CPIO file system into a run
time option. The build time configuration option is still used to decide whether to
use the new interface to populate with the default cpio archive symbols

libsel4muslcsys: Explicitly mark free fds

Marking free fds with a unique filetype allows for going from any arbitrary FD and
working out whether it is currently in use or not. Otherwise there is no way to check
if 'close' has been called on an FD and whether or not it is still valid

libsel4muslcsys: Make file descriptor interface public

When implementing additional layers of syscalls it can be useful to take advantage
of the existing file descriptor management. This commit makes the interface for this
public.

libsel4muslcsys: Add support for openat syscall

muslcsys: (trivial) remove trailing whitespace

muslcsys: remove duplicated constants

all: s/LOG_*/ZF_LOG*

Fix: Remove 'unused' from a variable that is used.

Actually, technically it's not, but ZF_LOG takes care of suppressing the
compiler warning for us.