[devtools] script support for managing source file copyright headers

Three subcommands to this script:

1) ./copyright_header.py report

Examines git-tracked files with extensions that match:

INCLUDE = ['*.h', '*.cpp', '*.cc', '*.c', '*.py']

Helps to:

-> Identify source files without copyright
-> Identify source files added with something other than "The Bitcoin Core
developers" holder so we can be sure it is appropriate
-> Identify unintentional typos in the copyright line

2) ./copyright_header.py update

Replaces fix-copyright-headers.py. It does file editing in native python
rather than subprocessing out to perl as was the case with
fix-copyright-headers.py. It also shares code with the 'report' functions.

3) ./copyright_header.py insert

Inserts a copyright header into a source file with the proper format and
dates.

contrib/devtools/README.md

@@ -24,21 +24,64 @@ the script should be called from the git root folder as follows.
 ```
 git diff -U0 HEAD~1.. | ./contrib/devtools/clang-format-diff.py -p1 -i -v
 ```
+copyright\_header.py
+====================
 
-fix-copyright-headers.py
-========================
+Provides utilities for managing copyright headers of `The Bitcoin Core
+developers` in repository source files. It has three subcommands:
 
-Every year newly updated files need to have its copyright headers updated to reflect the current year.
-If you run this script from the root folder it will automatically update the year on the copyright header for all
-source files if these have a git commit from the current year.
+```
+$ ./copyright_header.py report <base_directory> [verbose]
+$ ./copyright_header.py update <base_directory>
+$ ./copyright_header.py insert <file>
+```
+Running these subcommands without arguments displays a usage string.
 
-For example a file changed in 2015 (with 2015 being the current year):
+copyright\_header.py report \<base\_directory\> [verbose]
+---------------------------------------------------------
 
-```// Copyright (c) 2009-2013 The Bitcoin Core developers```
+Produces a report of all copyright header notices found inside the source files
+of a repository. Useful to quickly visualize the state of the headers.
+Specifying `verbose` will list the full filenames of files of each category.
 
-would be changed to:
+copyright\_header.py update \<base\_directory\> [verbose]
+---------------------------------------------------------
+Updates all the copyright headers of `The Bitcoin Core developers` which were
+changed in a year more recent than is listed. For example:
+```
+// Copyright (c) <firstYear>-<lastYear> The Bitcoin Core developers
+```
+will be updated to:
+```
+// Copyright (c) <firstYear>-<lastModifiedYear> The Bitcoin Core developers
+```
+where `<lastModifiedYear>` is obtained from the `git log` history.
 
-```// Copyright (c) 2009-2015 The Bitcoin Core developers```
+This subcommand also handles copyright headers that have only a single year. In
+those cases:
+```
+// Copyright (c) <year> The Bitcoin Core developers
+```
+will be updated to:
+```
+// Copyright (c) <year>-<lastModifiedYear> The Bitcoin Core developers
+```
+where the update is appropriate.
+
+copyright\_header.py insert \<file\>
+------------------------------------
+Inserts a copyright header for `The Bitcoin Core developers` at the top of the
+file in either Python or C++ style as determined by the file extension. If the
+file is a Python file and it has  `#!` starting the first line, the header is
+inserted in the line below it.
+
+The copyright dates will be set to be `<year_introduced>-<current_year>` where
+`<year_introduced>` is according to the `git log` history. If
+`<year_introduced>` is equal to `<current_year>`, it will be set as a single
+year rather than two hyphenated years.
+
+If the file already has a copyright for `The Bitcoin Core developers`, the
+script will exit.
 
 git-subtree-check.sh
 ====================