Small changes for Ubuntu Linux and MSWindows10 with Cygwin

[wedgingt]

These changes support building on Ubuntu Linux (as noted) and on Cygwin on MSWindows10 and avoid some compiler warnings related to integer types vs. printf() format specifiers by using C++ stream operators for output. Also, errno is being set to 138 on MS10/Cygwin before command line parsing, so I've added 'errno = 0;' just inside the start of that loop in mfaktc.c.

Both mfakto executables pass the internal (no command line options) self tests; no computation code is modified.

Let me know if there are any issues on other systems; I only have those two OSs to try this on and only have a GPU card mfakto can use on a Cygwin with MSWindows10 system.