Use correct config parser for MountAPIVFS (#18501)

Use correct config parser for MountAPIVFS (#18501)

As far as I can see, at some point the parser function for MountAPIVFS
was changed from the generic bool parser to a custom implementation, to
allow the context to keep track of whether MountAPIVFS had been set
explicitly. If not, exec_context_get_effective_mount_apivfs would fall
back to a default value. However, the corresponding entry in the big
parser table wasn't updated, meaning that the old bool parser was still
used, meaning that context->mount_apivfs_set remained at its default
value of false, meaning that the default value was always used and the
config option was effectively ignored.

Fix for 5e98086d1629f5c5b73645ba2568de4b09b7d958.

(cherry picked from commit 4d0052d52f1359a054e83037c733bb3330ba4124)