checksrc: add COPYRIGHTYEAR check

Forgetting to bump the year in the copyright clause when hacking has
been quite common among curl developers, but a traditional checksrc
check isn't a good fit as it would penalize anyone hacking on January
1st (among other things). This adds a more selective COPYRIGHTYEAR
check which intends to only cover the currently hacked on changeset.

The check for updated copyright year is currently not enforced on all
files but only on files edited and/or committed locally. This is due to
the amount of files which aren't updated with their correct copyright
year at the time of their respective commit.

To further avoid running this expensive check for every developer, it
adds a new local override mode for checksrc where a .checksrc file can
be used to turn on extended warnings locally.

Closes #3303
Reviewed-by: Daniel Stenberg <daniel@haxx.se>