Redundant exit() calls in las2las

[mloskot]

In the las2las.c, following pattern can be observed:

...
usage();
exit(0);

later in the code:

...
usage();
exit(1);

The exit() calls are inconsistent and redundant because usage() calls exit(0). I'd propose to remove exit(0) from usage() and call it on request after usage, with appropriate return code for particular context (error or success).

[hobu]

removed in r779. Fixed in the other utilitiees in r780