Convert use of ASSERT() to ASSERT0(), ASSERT3U(), ASSERT3S(), 
ASSERT3P(), and likewise for VERIFY().  In some cases it ended up 
making more sense to change the code, such as VERIFY on nvlist 
operations that I have converted to use fnvlist instead.  In one 
place I changed an internal struct member from int to boolean_t to 
match its use.  Some asserts that combined multiple checks with && 
in a single assert have been split to separate asserts, to make it 
apparent which check fails.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11971