Use dynamically-sized selected[] array for -P PATH

While at it, added a small optimization of not remembering
the path twice if it happens to be the same.

   text	   data	    bss	    dec	    hex	filename
 245111	    680	  10860	 256651	  3ea8b	strace_old
 245075	    680	   9804	 255559	  3e647	strace

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