repmgr: in standby switchover, quote file paths in remotely executed commands

Per suggestion from GitHub user sebasmannem (#229)

strutil.c

@@ -87,3 +87,34 @@ maxlen_snprintf(char *str, const char *format,...)
 
 	return retval;
 }
+
+
+/*
+ * Adapted from: src/fe_utils/string_utils.c
+ *
+ * Function not publicly available before PostgreSQL 9.6.
+ */
+void
+appendShellString(PQExpBuffer buf, const char *str)
+{
+	const char *p;
+
+	appendPQExpBufferChar(buf, '\'');
+	for (p = str; *p; p++)
+	{
+		if (*p == '\n' || *p == '\r')
+		{
+			fprintf(stderr,
+					_("shell command argument contains a newline or carriage return: \"%s\"\n"),
+					str);
+			exit(ERR_BAD_CONFIG);
+		}
+
+		if (*p == '\'')
+			appendPQExpBufferStr(buf, "'\"'\"'");
+		else
+			appendPQExpBufferChar(buf, *p);
+	}
+
+	appendPQExpBufferChar(buf, '\'');
+}