util: xatol() and xatoi() helpers introduced

These helpers are safe versions of atol() and atoi() respectively.
And they check for overflow and NAN errors

v3:
1) Added string print to convertion error message in xatol_base()
2) Added check for INT range in xatoi()

Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>