Generating a statistics file with r.terraflow (e.g. r.terraflow stats=stats.txt [...]) the first line in the stats file (is supposed to) state the amount of free memory:

head stats.txt 
Free Memory=-1
[0.0] Wed Jan 27 10:31:51 2021
Command Line: 
r.terraflow elevation=elev_srtm_30m@PERMANENT filled=filled_rast stats=stats.txt 
input elevation grid: elev_srtm_30m
output (filled_rast) elevations grid: filled_rast
[0.0] MFD flow direction
[0.0] D8CUT=999999986991104.000000
[0.0] Memory size: 300.00M (314572800) bytes
region size = 219.73K (225000) elts (450 rows x 500 cols)

with the value "-1" given if it failed to calculate the size.

Current implementation is based on getrlimit(), see: r.terraflow/stats.cpp and the value of rlimit.rlim_cur.

On a 64 bit Mac and probably Linux too (#712 (comment)) rlimit.rlim_cur == RLIM_INFINITY and the calculation fails. I may add that this code is already excluded from __MINGW32__ builds.

Part of the calculation is based on the deprecated sbrk(), originally reported with #712.

There is a lot of things to address with present code and all it does is printing one line in the stat file.

With this PR I propose to remove this part of the code, as I couldn't come up with a platform independent way to calculate "free memory" (also a vague concept considering virtual memory).

Fixes #712