Fixed endless loop for `p/x *(int *)0xf0000000'. kvm_uread() in
gdb was cloned from the buggy version of kvm_uread() in libkvm and
had the same bugs. It looped endlessly on EOF and checked errno
without setting it in the lseek() error check. The first bug caused
gdb to loop endlessly for reads from addresses between the end of
the user area and the start of the kernel text. kvm_uread() should
not be used for addresses beyond the end of the user area, but is
due to bugs elsewhere.