Avoid overwriting exit_status with a value indicating less important condition.

* src/tar.c (set_exit_status): New function.
* src/common.h (set_exit_status): New prototype.
* src/compare.c: Use set_exit_status instead of
exit_status assignments.
* src/create.c: Likewise.
* src/misc.c: Likewise.

* src/system.c (wait_for_grandchild): Use auto variable
instead of the global exit_status.
* src/incremen.c (scan_directory): Use file_removed_diag
instead of stat_diag.