os/osinit.c, os/utils.c: Exclude sigaction code when building for MinGW

MinGW doesn't have sigaction, so this patch is needed for building.

No attempt is made to actually install the fatal error signal handler, as MinGW
will simply terminate the process rather than deliver a fatal signal.

Also avoid using strsignal

Signed-off-by: Ryan Pavlik <rpavlik@iastate.edu>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Tested-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Reviewed-by: Keith Packard <keithp@keithp.com>

os/osinit.c

@@ -102,6 +102,7 @@ OsRegisterSigWrapper(OsSigWrapperPtr newSigWrapper)
  * OsSigHandler --
  *    Catch unexpected signals and exit or continue cleanly.
  */
+#if !defined(WIN32) || defined(__CYGWIN__)
 static void
 #ifdef SA_SIGINFO
 OsSigHandler(int signo, siginfo_t * sip, void *unused)
@@ -146,6 +147,7 @@ OsSigHandler(int signo)
     FatalError("Caught signal %d (%s). Server aborting\n",
                signo, strsignal(signo));
 }
+#endif /* !WIN32 || __CYGWIN__ */
 
 void
 OsInit(void)
@@ -155,6 +157,7 @@ OsInit(void)
     char fname[PATH_MAX];
 
     if (!been_here) {
+#if !defined(WIN32) || defined(__CYGWIN__)
         struct sigaction act, oact;
         int i;
 
@@ -181,6 +184,8 @@ OsInit(void)
                        siglist[i], strerror(errno));
             }
         }
+#endif /* !WIN32 || __CYGWIN__ */
+
 #ifdef HAVE_BACKTRACE
         /*
          * initialize the backtracer, since the ctor calls dlopen(), which