Fix a bug in dumpstr (no null termination). Essentially rewrote dumpstr

Fix a bug in dumpstr (no null termination). Essentially rewrote dumpstr

This is a 14 year old bug (!).

It wasn't biting us merely because outstr[80] was static, thus ended up
in bss and whatever was after it "accidentally" provided the NUL byte.
When dumpstr was changed to use on-stack buffer, the bug reared its ugly head.

This is a rewrite which is smaller and should be significantly faster
for _long_ strings.

   text	   data	    bss	    dec	    hex	filename
 244627	    680	  10860	 256167	  3e8a7	strace.t9/strace
 244563	    680	  10860	 256103	  3e867	strace.ta/strace

* util.c (dumpstr): Rewrite to be faster and smaller.

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