partitions: increase MAX_SEARCH_PARTITIONS and move to part.h

Move MAX_SEARCH_PARTITIONS to part.h so that functions in cmd
directory can find it.  At the same time, increase the value to
64 since some operating systems use many, and the resources
consumed by a larger value are minimal.

Changes since v6: none.

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>