WP-r52585: Check if the disk_free_space() exists

[bahiirwa]

Upgrade/Install: Check if the disk_free_space() function exists before calling it.

In PHP 8+, @ no longer suppresses fatal errors:

The @ operator will no longer silence fatal errors (E_ERROR, E_CORE_ERROR, E_COMPILE_ERROR, E_USER_ERROR, E_RECOVERABLE_ERROR, E_PARSE).

Reference: PHP 8: Backward Incompatible Changes.

Fixes #901

disk_free_space() may be disabled by hosts, which will throw a fatal error on a call to undefined function.

This change prevents the fatal error, and falls back to false when disk_free_space() is unavailable.

Follow-up to 25540, 25774, 25776, 25831, 25869.

Props costdev, jrf, swb1192, SergeyBiryukov. Fixes #54826. See #54730.

[mattyrob]

@bahiirwa - can you address the conflict?

[xxsimoxx]

Done some tests after the discussion on Slack.

In PHP < 8 when disk_free_space is disabled a warning is triggered (PHP Warning: disk_free_space() has been disabled for security reasons) and it returns null. In PHP 8 it's a fatal error: PHP Fatal error: Uncaught Error: Call to undefined function disk_free_space(), and the execution is stopped.

This mean that in PHP < 8 if disk_free_space is disabled the upgrade will continue. This mean that in PHP 8 if disk_free_space is disabled the upgrade will stop.

With this PR the behavior is consistent in all PHP versions. Also it's ok for SEMVER because the prior behavior can be considered a bug.