Simplify the getnode.cpp API - use bool returns, default paramaeter value

getnodedat() now performs a non-locked read by default.

Perform more checks of getnodedat() return value before calling putnodedat()
to avoid unintentionally zeroing out node.dab records.

Add/use unlocknodedat() method for unlocking a node.dab record without
writing.

Note: The userdat.c getnodedat() and putnodedat() still return int (i.e. 0 on
success), so that can be confusing.