pseudofs: Simplify pfs_readdir() and set eofflag

[markj]

Summary

There were two loops, one to skip entries before the requested offset,
and one to populate entries. Merge them and use the return value of
pfs_iterate() to decide whether to set eofflag.

As a side effect, this fixes a small bug: the first loop would decrement
offset to zero, and the second loop would increment it again.
However, offset was used to set d_off, and we should include the
starting offset there.

[kib]

sys/fs/pseudofs/pseudofs_vnops.c

888

This line definitely needs a comment that allproc_lock is required for pfs_iterate().
Although it is probably not needed for not-procdir directories.

[markj]

Add a comment noting the reason for acquiring allproc_lock.

For now I did not attempt to make it smarter.