Add correct trace indentation for several functions
Pre-declare for loop variables as local

This prevents them from overwriting variables from the enclosing scope,
as was happening to $dest in expire_src_snaps.
zfs get: specify -r before property name

Older version of the ZFS userland are particular about the order or
arguments.
Use -R rather than -r for age reporting

This frees up -r to use for 'recursive'.
Clarify log message when failing to acquire dest lock
Add -r option for recursive send

This allows the zfs-mirror:dest property to be used when it is
inherited, rather than only when explicitly set.

If the property has been overridden for a given dataset, this option
does *not* force it to be treated as if it were the same as the parent.