add methods to reuse sequences for saving disk space
add explicit test if we can make a link to reuse sequences and die if not
change default of using system perl to env perl if executable; minor code clean up
Revert "change default of using system perl to env perl if executable; minor code clean up"

This reverts commit 87daf11254bdd0c6c3914735136dacd207a23dfb.
Merge branch 'topic/reuse_sequences'