Print syscall names only for defined syscalls

Print syscall names only for defined syscalls

The string literal "__NR_syscall_4294967295" is semantically incorrect
as there is no such constant defined.

* syscall.c (syscall_name): Return NULL if there is no syscall
corresponding to the given number.
* defs.h (syscall_name): Document this behaviour.
* printsiginfo.c (print_si_info): Print syscall name with "__NR_" prefix
only if there is a syscall corresponding to si_syscall number; print
a plain syscall number otherwise.
* tests/ptrace.c (main): Update expected output.