Roll back "die on malloc failure" behaviour a bit

Roll back "die on malloc failure" behaviour a bit

After recent change, select(2^31-1, NULL, NULL, NULL)
would make strace exit. This change caps fdsize so that
it is always in [0, 1025*1024], IOW: we will try to allocate at most
1 megabyte, which in practice will almost always work,
unlike malloc(2Gig).

* desc.c (decode_select): Cap fdsize to 1024*1024.
* pathtrace.c (pathtrace_match): Cap fdsize to 1024*1024.
* file.c (sys_getdents): Cap len to 1024*1024.
(sys_getdents64): Cap len to 1024*1024.
* util.c (dumpiov): Refuse to process iov with more than 1024*1024
elements. Don't die on malloc failure.
(dumpstr): Don't die on malloc failure.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>