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 5e98086d.