cat: don’t trust st_size on /proc files

cat: don’t trust st_size on /proc files

* src/cat.c (main):
Improve test for when copying will exhaust the output device.
Do not rely on st_size, which is unreliable in /proc.
Use lseek instead; this is good enough here.
* tests/cat/cat-self.sh: Test the relaxation of the heuristic
for self-copying.