Fix fuse include paths for individual unixfs'.

ancientfs had the right path and compiles common/unixfs first, so it wasn't apparent when using the top level Makefile.
Build with the environment's CC and CFLAGS if they exist.
Fix build with clang -Wall -Werror [1/2]

Uninitialized read, these are unused leftovers from ancientfs_bcpio.
Fix build with clang -Wall -Werror [2/2]

i_block is unsigned (tautological compare).
Static analyzer fixes [1/4]

Prevents a double free.
Static analyzer fixes [2/4]

Potential garbage return from ufs init.
Static analyzer fixes [3/4]

Prevent uninitialized read in U_ufs_get_block when ufs_getfrag_block fails.
Static analyzer fixes [4/4]

Overread when checking for old-format ustar identifier ("ustar ").