Add method precheck to the g_part interface. The precheck

method allows schemes to reject the ctl request, pre-check
the parameters and/or modify/set parameters. There are 2
use cases that triggered the addition:
1.  When implementing a R/O scheme, deletes will still
    happen to the in-memory representation. The scheme is
    not involved in that operation. The pre-check method
    can be used to fail the delete up-front. Without this
    the write to disk will typically fail, but at that
    time the delete already happened.
2.  The EBR scheme uses a linked list to record slices.
    There's no index. The EBR scheme defines the index
    as a function of the start LBA of the partition. The
    add verb picks an index for the range and then invokes
    the add method of the scheme to fill in the blanks. It
    is too late for the add method to change the index.
    The pre-check is used to set the index up-front. This
    also (silently) overrides/nullifies any (pointless)
    user-specified index value.

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