Fix xgap redirection of errout

[ChrisJefferson]

This fixes xgap's special redirection of errout. Without this, xgap (and gap.app, which uses the same interface) don't correctly display break loops.

[codecov]

Codecov Report

Merging #2660 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2660      +/-   ##
==========================================
+ Coverage   75.19%   75.19%   +<.01%     
==========================================
  Files         478      478              
  Lines      242242   242243       +1     
==========================================
+ Hits       182144   182146       +2     
+ Misses      60098    60097       -1

Impacted Files	Coverage Δ
src/system.c	72.14% <0%> (-0.21%)	⬇️
src/iostream.c	62.35% <0%> (-1.15%)	⬇️
hpcgap/lib/hpc/stdtasks.g	64.96% <0%> (+1.27%)	⬆️