os400.c: fix ASSIGNWITHINCONDITION checksrc warnings

All occurrences of assignment within conditional expression in
os400sys.c rewritten into two steps: first assignment and then the check
on the success of the assignment. Also adjust related incorrect brace
positions to match project indentation style.

This was spurred by seeing "if((inp = input_token))", but while in there
all warnings were fixed.

There should be no functional change from these changes.

Closes #2525