Use /usr/bin/env python for virtualenv (#1025)

This helps with virtualenv although it can open
up another security hole if you end up using an
unexpected python interpreter.

Overall it seems to make sense to err on the side
of usability but it's good to be aware of security.

However, for the remaining utility scripts that require
python 2, we explicitly note this with #!/usr/bin/python2.

util/unpep8

@@ -1,7 +1,7 @@
-#!/usr/bin/python
+#!/usr/bin/python2
 
 """
-Translate from PEP8 Python style to Mininet (i.e. Arista-like) 
+Translate from PEP8 Python style to Mininet (i.e. Arista-like)
 Python style
 
 usage: unpep8 < old.py > new.py
@@ -51,7 +51,7 @@ def fixUnderscoreTriplet( match ):
 def reinstateCapWords( text ):
    underscoreTriplet = re.compile( r'[A-Za-z0-9]_[A-Za-z0-9]' )
    return underscoreTriplet.sub( fixUnderscoreTriplet, text )
- 
+
 def replaceTripleApostrophes( text ):
    "Replace triple apostrophes with triple quotes."
    return text.replace( "'''", '"""')
@@ -60,7 +60,7 @@ def simplifyTripleQuotes( text ):
    "Fix single-line doc strings."
    r = re.compile( r'"""([^\"\n]+)"""' )
    return r.sub( r'"\1"', text )
-   
+
 def insertExtraSpaces( text ):
    "Insert extra spaces inside of parentheses and brackets/curly braces."
    lparen = re.compile( r'\((?![\s\)])' )
@@ -76,9 +76,9 @@ def insertExtraSpaces( text ):
    lcurly = re.compile( r'\{(?![\s\}])' )
    text = lcurly.sub( r'{ ', text )
    rcurly = re.compile( r'([^\s\{])(?=\})' )
-   text = rcurly.sub( r'\1 ', text)   
+   text = rcurly.sub( r'\1 ', text)
    return text
-   
+
 def fixDoxygen( text ):
    """Translate @param foo to foo:, @return bar to returns: bar, and
       @author me to author: me"""
@@ -96,11 +96,11 @@ def removeCommentFirstBlankLine( text ):
    "Remove annoying blank lines after first line in comments."
    line = re.compile( r'("""[^\n]*\n)\s*\n', re.MULTILINE )
    return line.sub( r'\1', text )
-   
+
 def fixArgs( match, kwarg = re.compile( r'(\w+) = ' ) ):
    "Replace foo = bar with foo=bar."
    return kwarg.sub( r'\1=', match.group() )
-   
+
 def fixKeywords( text ):
    "Change keyword argumentsfrom foo = bar to foo=bar."
    args = re.compile( r'\(([^\)]+)\)', re.MULTILINE )
@@ -115,7 +115,7 @@ def fixKeywords( text ):
 def lineIter( text ):
    "Simple iterator over lines in text."
    for line in text.splitlines(): yield line
-   
+
 def stringIter( strList ):
    "Yield strings in strList."
    for s in strList: yield s
@@ -134,7 +134,7 @@ def restoreRegex( regex, old, new ):
 # This is a cheap hack, and it may not work 100%, since
 # it doesn't handle multiline strings.
 # However, it should be mostly harmless...
-   
+
 def restoreStrings( oldText, newText ):
    "Restore strings from oldText into newText, returning result."
    oldLines, newLines = lineIter( oldText ), lineIter( newText )
@@ -149,13 +149,13 @@ def restoreStrings( oldText, newText ):
       newLine = restoreRegex( tickStrings, oldLine, newLine )
       result += newLine + '\n'
    return result
-   
+
 # This might be slightly controversial, since it uses
 # three spaces to line up multiline comments. However,
 # I much prefer it. Limitations: if you have deeper
 # indents in comments, they will be eliminated. ;-(
 
-def fixComment( match, 
+def fixComment( match,
    indentExp=re.compile( r'\n([ ]*)(?=[^/s])', re.MULTILINE ),
    trailingQuotes=re.compile( r'\s+"""' ) ):
    "Re-indent comment, and join trailing quotes."
@@ -166,17 +166,17 @@ def fixComment( match,
    if len( originalIndent ) is not 0: indent += '   '
    comment = indentExp.sub( indent, comment )
    return originalIndent + trailingQuotes.sub( '"""', comment )
-   
+
 def fixCommentIndents( text ):
    "Fix multiline comment indentation."
    comments = re.compile( r'^([ ]*)("""[^"]*""")$', re.MULTILINE )
    return comments.sub( fixComment, text )
- 
+
 def removeBogusLinefeeds( text ):
    "Remove extra linefeeds at the end of single-line comments."
    bogusLfs = re.compile( r'"([^"\n]*)\n"', re.MULTILINE )
    return bogusLfs.sub( '"\1"', text)
-   
+
 def convertFromPep8( program ):
    oldProgram = program
    # Program text transforms