fix issues with generating the string for new paths when renaming dirs

* char[] is like a char* so no need for & as in &new_path
* ensure that the last strncat does not overrun its buffer
fix all "pointer targets in assignment differ in signedness" warnings

sqlite3_column_text() returns a const unsigned char* to be extra careful
when supporting all platforms, since char is not guaranteed to be unsigned
but we only care about sane platforms where char is unsigned, so just cast
to const char*
comment out all xattr stubs, we don't use xattrs nor need to implement them
in get_value(), init size to 0 in case the database is busy

This also gets rid of this warning:
sqlfs.c:1482:32: error: 'size' may be used uninitialized in this function
fix compiler warnings in the tests