indent: Fix buffer overflow

[des]

Summary

The function used to create a backup of the input before starting work
used a static buffer and did not check that the file name it constructed
did not overflow. Switch to using asprintf(), clean up the rest of the
function, and update some comments that still referred to an earlier
version of the code.

MFC after: 1 week
Sponsored by: Klara, Inc.

[bnovkov]

usr.bin/indent/indent.c

1270

Shouldn't we free bakfile before returning?

[des]

usr.bin/indent/indent.c

1270

yeah, it used to be global, but there's no reason to keep it around.

[des]

free