Allow hookspecs to declare default values for arguments

[bluetech]

This is mainly a backward compatibility feature, to allow a hookspec to add new parameters to an existing hook without breaking existing callers.

The immediate motivation for this is coming from pytest-dev/pytest#15035, where pytest wants to add an argument to pytest_deselect hook but there are many existing external callers.

This feature has been discussed and implemented before:

• support defaults to enable deprecation #15
• Support defaults via kwargs #43
• Howto handle hook changes #170

although I haven't been able to figure out why it was rejected. To me it seems like an intuitive and natural feature, so I decided to put forward another attempt :)

The call overhead when default is not declared is a function call + 2 ifs, which seems OK to me. (Potentially can inline _apply_defaults but I still hold hope that cpython will one day do such trivial optimizations itself).

Note: there is a separate matter with defaults declared by hookimpls, which I think currently behave in an unintuitive way. There's an issue for it #442. I will try to tackle it next, as it has some interaction with this feature (see test_does_not_override_hookimpl_default test in this PR), but it's preexisting.

Fix #170.

[RonnyPfannschmidt]

my key nitpick form last time is that i only ever wanted to allow none as sentinel/default in order to prevent some types of drift

but i recon that this might have been too pedantic