Add some file functions

popen, pclose, fgetc and fflush
Merge pull request #908 from david-christiansen/ext-help

Document -X at command line
Merge branch 'master' of github.com:idris-lang/Idris-dev
Better environment management in unification

Also properly guard !!s so that if there is a scope error, you get an
error message rather than a crash (though no guarantees the error
message will be much use...)

Fixes #738, #600 and #891 (and possibly more...)
Merge remote-tracking branch 'upstream/master' into ideslave-explicit-namespaces
Add missing output annotation serialization to ideslave