basic/process-util: add mode where posix shell escape is used for quoting

The new flag is not used, except in tests, so no functional change yet.

This way, the command as shown can be copied-and-pasted into the shell
in more cases. For simple cases, shell quoting with "" is enough. But
$'' is needed when there are control characters in the command.

src/basic/process-util.h

@@ -36,6 +36,7 @@ typedef enum ProcessCmdlineFlags {
         PROCESS_CMDLINE_COMM_FALLBACK = 1 << 0,
         PROCESS_CMDLINE_USE_LOCALE    = 1 << 1,
         PROCESS_CMDLINE_QUOTE         = 1 << 2,
+        PROCESS_CMDLINE_QUOTE_POSIX   = 1 << 3,
 } ProcessCmdlineFlags;
 
 int get_process_comm(pid_t pid, char **name);