Change how printf parses its format string

[suyuee]

Currently printf does not allow string format specifiers to be directly
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 PR 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

[danobi]

Nice fix, lgtm.

Will merge after release tomorrow