Rename context structure variable
Add reentrant version of ny_error function
Determine system page size on initialisation
Implement memory allocator
alloc: Introduce sentinel element

Instead of tracking the number of free objects, introduce a sentinel value
to mark the tail of the free list.
alloc: Debug object release
tcp: Cumulated fixes for minor errors
makefile: Warn about signed integer overflow
Mark functions as const and pure where applicable