Shrink space needed by undefined syscalls in syscall tables.

Shrink space needed by undefined syscalls in syscall tables.

Undefined syscall looked like this before this change:
{ 5,    0,      printargs,              "SYS_53"                },
That is, "SYS_53" string had to be allocated and stored in strace binary.
Since now SCNO_IN_RANGE() macro requires sysent[scno].sys_func != NULL
for valid syscalls, we can replace printargs with NULL in such lines
and make them "invalid", thus not requiring syscall name string.

Savings on i386:
   text	   data	    bss	    dec	    hex	filename
 237389	    704	  18944	 257037	  3ec0d	strace.before
 236973	    704	  18944	 256621	  3ea6d	strace
Savings on mips:
 336551  153692   38320  528563   810b3 strace.before
 275543  153688   38320  467551   7225f strace

Tested to still decode undefined syscalls correctly (syscall no. 222 on i386).

* linux/*/syscallent.h: Replace 'printargs, "SYS_nnn"' with
'NULL, NULL'.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>