[flang][runtime] Reset the left tab limit when flushing output

When flushing output to a non-positionable tty or socket file, reset the
left tab limit.  Otherwise, non-advancing output to that file will contain
an increasing amount of leading spaces in each flush.  Also, detect
newline characters in stream output, and treat them as record
advancement.

Differential Revision: https://reviews.llvm.org/D148157