…ctly

followed by any alphabetic characters since it would take all the following
letters as part of the specifiers. As an example:

```
bpftrace -e 'BEGIN { @start = nsecs; } i:s:1 { printf("Elapsed: %ds\n", (nsecs - @start)/1000000000); }'
stdin:1:35-95: ERROR: printf: Unknown format string token: %ds
BEGIN { @start = nsecs; } i:s:1 { printf("Elapsed: %ds\n", (nsecs - @start)/1000000000); }
```
I think it's good to fix it since I could not think of any obvious downsides.
This works to address this issue by changing the way `printf` parse the
format strings. It would also affect `cat` and `system` as well. With this PR:

```
bpftrace -e 'BEGIN { @start = nsecs; } i:s:1 { printf("Elapsed: %ds\n", (nsecs - @start)/1000000000); }'
Attaching 2 probes...
Elapsed: 1s
Elapsed: 2s
Elapsed: 3s
Elapsed: 4s
Elapsed: 5s
^C

@start: 2976003477708
```