Test for overflow when translating task periods/intervals to integer variable.

partial fix to issue #76 - calculate task periods with 1ms (instead of 1ns) resolution. Allows for tasks with periods larger than 4.295s, but smaller than 50 days.

partial fix for issue 74: allow deref of REF in left hand side of assignment (only works in FUNCTIONS for now)

fix issue #78: IL NOT operator not working

fix issue#77 - when IL FB followed by SFC FB, non-existant error was detected in IL code.

revert commits improved performance of some extensible Standard Functions (ADD, MUL, AND, OR, XOR)

Following commits are reverted:
mjsousa 0b275a2 improve performance of some extensible Standard Functions (ADD, MUL, AND, OR, XOR) -- increase hardcoded limit to 499
mjsousa 2228799 improve performance of some extensible Standard Functions (ADD, MUL, AND, OR, XOR) -- Add comments!!
mjsousa ce81fa6 improve performance of some extensible Standard Functions (ADD, MUL, AND, OR, XOR)"

The reason is that they cause regression in some cases (if function is
used as argument for function block, for example) and this is not
fixed for a long time.

merge

fix backup/restore functions: now also backup/restore programs instantiated to run inside tasks.

add include <time.h>, required for time_t

improve performance of some extensible Standard Functions (ADD, MUL, AND, OR, XOR) -- increase hardcoded limit to 499