Fix lame kernel version checking code

Fix lame kernel version checking code

The code "os_release[0] >= '3'" is not good for any
finer-grained checks such as "kernel >= 3.2.1".
Let's proactively fix it.

* strace.c: Change os_release from string to integer.
(get_os_release): Parse uname.release to KERNEL_VERSION
representation.
(init): Convert kernel version check to KERNEL_VERSION.

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