C++11 fix: Avoid casting the SIG_* function-types to (in theory) constant uintptr_t values.

Clang complains about the use of SIG_DFL/SIG_IGN in a switch case; while
they are implemented as integer constants cast to function pointers,

Our casting them back for use in a switch() statement is not strictly
well-defined, so I've switched them to the more correct if () statements.

Issue: PLCR-538