core: Handle disappearing managed dataspaces

This patch adds the consideration of suddenly disappearing managed
dataspaces in the 'Rm_session_component::reverse_lookup' function.
Previously, this case resulted in a seamingly valid translation.

Fixes #701
Add ports-foc to build.conf for foc_arndale
lwip: Print IP address in release mode
heap: Reduce maximum chunk size to 1 MiB on 32bit

With the previous maximum, the heap would allocate chunks of up to 4
MiB, which may lead to overly large slack memory.
terminal_mux: Free ncurses meta data