ls: make readlink error to not disrupt output (try ls -l /proc/self/fd).

ls: make readlink error to not disrupt output (try ls -l /proc/self/fd).
libbb: make xmalloc_readlink_or_warn warning more specific.

function                                             old     new   delta
xmalloc_readlink_or_warn                              33      61     +28
showfiles                                           1495    1460     -35
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(add/remove: 0/0 grow/shrink: 1/1 up/down: 28/-35)             Total: -7 bytes