statfs: avoid static allocation Previously we have "static struct statfs" to return a string. That is not multiple-thread safe. This patch moves the allocation to the caller to address the problem.

statfs: change Linux feature detection Previously we check three things: "statfs" function, "linux/magic.h" and "sys/vfs.h" headers. But we didn't check "struct statfs" or the "f_type" field. That means if a system has "statfs" but "struct statfs" is not defined in the two header files we check, or defined without the "f_type" field, the compilation will fail. This patch combines the checks (2 headers + 1 function + 1 field) together and sets "HAVE_LINUX_STATFS". It makes setup.py faster (less checks), and more reliable (immutable to the issue above).

rebase: don't require destination if commands.rebase.requiredest=False