run-command: move check_pipe() from write_or_die to run_command
Move check_pipe() to run_command and make it public. This is necessary
to call the function from pkt-line in a subsequent patch.
While at it, make async_exit() static to run_command.c as it is no
longer used from outside.
Signed-off-by: Lars Schneider <larsxschneider@gmail.com> Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>