Export READ_ALL_COMMANDS via header file

[markuspf]

Needed for #2702, though I am a bit reluctant to give this function more credibility by exporting it through a header file.

Also separate FuncREAD_ALL_COMMANDS (exported to GAP level) and READ_ALL_COMMANDS (kernel function) for conistency.

[codecov]

Codecov Report

Merging #2707 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2707      +/-   ##
==========================================
+ Coverage   75.52%   75.52%   +<.01%     
==========================================
  Files         478      478              
  Lines      241620   241621       +1     
==========================================
+ Hits       182488   182492       +4     
+ Misses      59132    59129       -3

Impacted Files	Coverage Δ
src/streams.c	77.6% <100%> (+0.02%)	⬆️
src/iostream.c	62.35% <0%> (-1.15%)	⬇️
src/objset.c	84.59% <0%> (-0.23%)	⬇️
src/stats.c	89.62% <0%> (+0.19%)	⬆️
hpcgap/lib/hpc/stdtasks.g	64.96% <0%> (+1.53%)	⬆️