Enable itimer cancellation

[mikbras]

The itimer cancellation feature was implemented as part of an earlier PR but then conditionally commented out because it was unrelated to that PR.

Create a new PR by removing the following directives (leaving the source lines between the directives).

#ifdef ITIMER_CANCELLATION /* See PR-262 */
...
#endif

[paulcallen]

this needs to be done because it shows up as a leak with debug malloc