fusefs: more build fixes

• Fix printf format strings on 32-bit OSes
• Fix -Wclass-memaccess violation on GCC-8 caused by using memset on an object of non-trivial type.
• Fix memory leak in MockFS::init
• Fix -Wcast-align error on i386 in expect_readdir
• Fix some heterogenous comparison errors on 32-bit OSes.

Sponsored by: The FreeBSD Foundation