Respect SIMPLE_BACKUP_SUFFIX environment variable in indent(1)

Instead of using a non-configurable ".BAK" suffix, respect the
SIMPLE_BACKUP_SUFFIX environment variable also used by patch(1). This
simplifies cleanup operations in some patch/indent workflows.

Reviewed by: cem (earlier version), emaste, pstef
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D10921