Even though arpa/inet.h is "networky" and this isn't a network
application, arpa/inet.h is in POSIX whereas endian.h is non-standard.
This should let git-crypt build on Mac OS X.
Although glibc's implementation of mkstemp creates temporary files with
a safe (i.e. 0600) mode, POSIX does not mandate any particular mode. So
to ensure maximum cross-platform safety, we must set a umask of 0077
before calling mkstemp.
