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.c

@@ -208,7 +208,10 @@ int get_process_cmdline(pid_t pid, size_t max_columns, ProcessCmdlineFlags flags
         if (full < 0)
                 return full;
 
-        if (flags & PROCESS_CMDLINE_QUOTE) {
+        if (flags & (PROCESS_CMDLINE_QUOTE | PROCESS_CMDLINE_QUOTE_POSIX)) {
+                ShellEscapeFlags shflags = SHELL_ESCAPE_EMPTY |
+                        FLAGS_SET(flags, PROCESS_CMDLINE_QUOTE_POSIX) * SHELL_ESCAPE_POSIX;
+
                 assert(!(flags & PROCESS_CMDLINE_USE_LOCALE));
 
                 _cleanup_strv_free_ char **args = NULL;
@@ -220,7 +223,7 @@ int get_process_cmdline(pid_t pid, size_t max_columns, ProcessCmdlineFlags flags
                 for (size_t i = 0; args[i]; i++) {
                         char *e;
 
-                        e = shell_maybe_quote(args[i], SHELL_ESCAPE_EMPTY);
+                        e = shell_maybe_quote(args[i], shflags);
                         if (!e)
                                 return -ENOMEM;