Make shebangs portable and prefix friendly
See also: https://blogs.gentoo.org/mgorny/2016/02/08/a-quick-note-on-portable-shebangs/

--- a/scripts/last-dotplot.py
+++ b/scripts/last-dotplot.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Read pair-wise alignments in MAF or LAST tabular format: write an
 # "Oxford grid", a.k.a. dotplot.
--- a/scripts/last-map-probs.py
+++ b/scripts/last-map-probs.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright 2010, 2011, 2012 Martin C. Frith
 
--- a/scripts/last-merge-batches.py
+++ b/scripts/last-merge-batches.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright 2010, 2011 Martin C. Frith
 
--- a/scripts/last-pair-probs.py
+++ b/scripts/last-pair-probs.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright 2011, 2012, 2013 Martin C. Frith
 
--- a/scripts/last-reduce-alignments.sh
+++ b/scripts/last-reduce-alignments.sh
@@ -1,4 +1,4 @@
-#! /bin/sh
+#!/usr/bin/env sh
 
 # This script reads MAF-format alignments with lastal header
 # information, removes "uninteresting" alignments, and writes the
--- a/scripts/last-remove-dominated.py
+++ b/scripts/last-remove-dominated.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Read MAF-format alignments, and write those are not "dominated" by
 # any other one.  X dominates Y if they overlap on the top sequence,
--- a/scripts/last-split-probs.py
+++ b/scripts/last-split-probs.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright 2012 Martin C. Frith
 
--- a/scripts/maf-convert.py
+++ b/scripts/maf-convert.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 # Copyright 2010, 2011 Martin C. Frith
 # Read MAF-format alignments: write them in other formats.
 # Seems to work with Python 2.x, x>=4
--- a/scripts/maf-cull.py
+++ b/scripts/maf-cull.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Read MAF-format alignments.  Write them, omitting alignments whose
 # coordinates in the top-most sequence are contained in those of >=
--- a/scripts/maf-join.py
+++ b/scripts/maf-join.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright 2009, 2010, 2011 Martin C. Frith
 
--- a/scripts/maf-sort.sh
+++ b/scripts/maf-sort.sh
@@ -1,4 +1,4 @@
-#! /bin/sh
+#!/usr/bin/env sh
 
 # Sort MAF-format alignments by sequence name, then strand, then start
 # position, then end position, of the top sequence.  Also, merge
--- a/scripts/maf-swap.py
+++ b/scripts/maf-swap.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#!/usr/bin/env python2
 
 # Read MAF-format alignments, and write them, after moving the Nth
 # sequence to the top in each alignment.