scripts: allow all perl scripts to be run directly

- Enable execute permission (chmod +x)

- Change interpreter to /usr/bin/env perl

Closes https://github.com/curl/curl/pull/2222

docs/cmdline-opts/gen.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 
 =begin comment
 

docs/examples/adddocsref.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 
 # pass files as argument(s)
 

docs/libcurl/mksymbolsmanpage.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 # ***************************************************************************
 # *                                  _   _ ____  _
 # *  Project                     ___| | | |  _ \| |

docs/libcurl/symbols.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 #***************************************************************************
 #                                  _   _ ____  _
 #  Project                     ___| | | |  _ \| |