Cleans up os_daemon() in Windows case.

This provides an always-failing os_daemon() for platforms (e.g.,
Windows) that can't provide a real daemon() call.  This is more
consistent than simply omitting the function.

It also bases the condition on the lack of fork(), rather than the
presence of winsock2.h.

TESTED:
Ran "scons build-all check" on OSX, Linux, FreeBSD, OpenBSD, and
NetBSD.  Verified that HAVE_FORK is set in all cases.

SConstruct

@@ -758,7 +758,7 @@ else:
 
     # check function after libraries, because some function require libraries
     # for example clock_gettime() require librt on Linux glibc < 2.17
-    for f in ("daemon", "strlcpy", "strlcat", "clock_gettime", "strptime", "gmtime_r", "inet_ntop", "fcntl"):
+    for f in ("daemon", "strlcpy", "strlcat", "clock_gettime", "strptime", "gmtime_r", "inet_ntop", "fcntl", "fork"):
         if config.CheckFunc(f):
             confdefs.append("#define HAVE_%s 1\n" % f.upper())
         else:

os_compat.c

@@ -45,10 +45,10 @@ int clock_gettime(clockid_t clk_id, struct timespec *ts)
 /* End of clock_gettime section */
 
 #ifndef HAVE_DAEMON
-#ifndef HAVE_WINSOCK2_H
-/* No daemon() provided for Windows as not currently needed */
 /* Simulate Linux/BSD daemon() on platforms that don't have it */
 
+#ifdef HAVE_FORK
+
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -91,7 +91,19 @@ int os_daemon(int nochdir, int noclose)
     /* coverity[leaked_handle] Intentional handle duplication */
     return 0;
 }
-#endif /* HAVE_WINSOCK2_H */
+#else /* !HAVE_FORK */
+
+#include <errno.h>
+
+int os_daemon(int nochdir, int noclose)
+{
+    (void) nochdir; (void) noclose;
+    errno = EINVAL;
+    return -1;
+}
+
+#endif /* !HAVE_FORK */
+
 #else /* HAVE_DAEMON */
 
 #ifdef __linux__