Implement fstat_c() and lseek_c() methods for the cheri file-descriptor class.
Add a top-level comment for cheri_enter.c.
Include cheri_invoke.h for cheri_invoke().
Don't include cheri_invoke.h in preference for local definition.
Avoid using __output capabilities for cheri_fd method arguments for now.

Use 'fd_object' rather than 'co' as the argument name.
Implement further fd_object methods.

Hook up cheri_fd_stub.c to the libc_cheri build.