Filesystem API: Introduce pre_unzip_file and post_unzip_file hooks.

[costdev]

This introduces two new actions: pre_unzip_file and post_unzip_file.

Both actions pass the following:

• string $file - Full path and filename of ZIP archive.
• string $to - Full path on the filesystem to extract archive to.
• string[] $needed_dirs - A full list of required folders needed to be created.
• float|false $required_space - The space required to unzip the file and copy its contents, with a 10% buffer. False if disk_free_space() returned false.
• string $mode - The mode used to unzip. Accepts "ziparchive" or "pclzip". Default "ziparchive".

Additionally:

• Combines the _unzip_file_ziparchive() and _unzip_file_pclzip() functions into a new function: _unzip_file().
  • Conditionally runs distinct functionality, and merges common functionality.
  • For BC, both functions act as wrappers for _unzip_file( $file, $to, $needed_dirs, ziparchive||pclzip ).
  • Note: Despite the docblock clearly stating that these functions should not be used directly, plugins still use it directly.
• Implements _unzip_file() in unzip_file() to avoid calling the wrapper functions unnecessarily.
  • No other locations in Core use _unzip_file_ziparchive() or _unzip_file_pclzip() directly.

Trac ticket: https://core.trac.wordpress.org/ticket/37719