Update, rename and clean up various scripts

Rename contrib/tools/chkfmt to contrib/tools/chkfmt.sh; refactor:
 * Increase the max line length for contrib/**/*.[ch] from 96 to 100.
 * Set the max line length for ci_*.sh to 100.
 * Use `basename $0` instead of the hard-coded script name.
 * Update comments.

Remove contrib/tools/reindent. For automated code formatting, including
indentation, we need a robust solution.

Add an empty line after the hashbang line in all scripts.

Remove the "last changed" version info from comment headers.
(The version control system maintains this information automatically.)

scripts/checksym.awk

@@ -1,4 +1,5 @@
 #!/bin/awk -f
+
 # Check a list of symbols against the master definition
 # (official) list.  Arguments:
 #

scripts/dfn.awk

@@ -1,8 +1,7 @@
 #!/bin/awk -f
+
 # scripts/dfn.awk - process a .dfn file
 #
-# last changed in libpng version 1.5.19 - August 21, 2014
-#
 # Copyright (c) 2013-2014 Glenn Randers-Pehrson
 #
 # This code is released under the libpng license.

scripts/options.awk

@@ -1,8 +1,7 @@
 #!/bin/awk -f
+
 # scripts/options.awk - library build configuration control
 #
-# last changed in libpng version 1.6.11 - June 5, 2014
-#
 # Copyright (c) 1998-2014 Glenn Randers-Pehrson
 #
 # This code is released under the libpng license.
@@ -243,7 +242,7 @@ $1 == "file" && NF >= 2{
 
 # option NAME ( (requires|enables|if) NAME* | on | off | disabled |
 #                sets SETTING VALUE+ )*
-#     
+#
 #   Declares an option 'NAME' and describes its default setting (disabled)
 #   and its relationship to other options.  The option is disabled
 #   unless *all* the options listed after 'requires' are set and at