Fix the MSVC build on MS-Windows.

 output.c (vsnprintf) [_MSC_VER]: Define, instead of defining
 snprintf, which isn't used.  Reported by Christian Boos
 <cboos@edgewall.org>.
 NMakefile.template (OBJS): Add load.obj and posixfcn.obj.
 ($(OUTDIR)/pathstuff.obj): New dependency.
 Suggested by Christian Boos <cboos@edgewall.org>.
Fix MinGW64 problem with non-compliant vsnprintf.

 makeint.h (__USE_MINGW_ANSI_STDIO) [__MINGW64_VERSION_MAJOR]:
 Define for MinGW64, to force it to use an ANSI-compliant
 implementation of vsnprintf.  Reported by Christian Boos
 <cboos@edgewall.org>.
Fix initialization of stringlist variables for jobserver_fds and sync_mutex.

 (tiny change)

 main.c (prepare_mutex_handle_string, main): Initialize
 stringlist variables with at least 2 members, as one member is not
 currently supported.