Split into multiple files

For multiple reasons the program is now split:
- Make future porting to OpenBSD easier
- Assign header includes to individiual functions
- Make future program extensions easier
- Recompile only changed parts

run_command.c

@@ -0,0 +1,26 @@
+#include <err.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "util.h"
+
+const char *
+run_command(const char *cmd)
+{
+	char *p;
+	FILE *fp;
+
+	fp = popen(cmd, "r");
+	if (fp == NULL) {
+		warn("Failed to get command output for %s", cmd);
+		return NULL;
+	}
+	p = fgets(buf, sizeof(buf) - 1, fp);
+	pclose(fp);
+	if (!p)
+		return NULL;
+	if ((p = strrchr(buf, '\n')) != NULL)
+		p[0] = '\0';
+
+	return buf[0] ? buf : NULL;
+}