Update imsg*.[ch] from OpenBSD, add some compat bits it needs and remove some

bits it doesn't.

compat/getdtablecount.c

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2017 Nicholas Marriott <nicholas.marriott@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
+ * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <sys/types.h>
+
+#include <glob.h>
+#include <unistd.h>
+
+#include "compat.h"
+
+void fatal(const char *, ...);
+void fatalx(const char *, ...);
+
+#ifdef HAVE_PROC_PID
+int
+getdtablecount(void)
+{
+	char	path[PATH_MAX];
+	glob_t	g;
+	int	n;
+
+	if (snprintf(path, sizeof path, "/proc/%ld/fd/*", (long)getpid()) < 0)
+		fatal("snprintf overflow");
+	switch (glob(path, 0, NULL, &g)) {
+	case GLOB_NOMATCH:
+		return (0);
+	case 0:
+		break;
+	default:
+		fatal("glob(\"%s\") failed", path);
+	}
+	n = g.gl_pathc;
+	globfree(&g);
+	return (n);
+}
+#else
+int
+getdtablecount(void)
+{
+	return (0);
+}
+#endif