proc-cmdline: introduce proc_cmdline_strv()

When we are running in a container, we parse the command line of PID1 in
proc_cmdline_parse() or friends. Previously, first we merge the command
line nulstr as a single string, and then split by using
extract_first_word(). That's not only redundant, but also unsafe when
the command line argument contain a space.

This drops the redundant steps, hence we can safely parse arguments with
space.

src/basic/proc-cmdline.h

@@ -15,6 +15,7 @@ typedef enum ProcCmdlineFlags {
 typedef int (*proc_cmdline_parse_t)(const char *key, const char *value, void *data);
 
 int proc_cmdline(char **ret);
+int proc_cmdline_strv(char ***ret);
 
 int proc_cmdline_parse(const proc_cmdline_parse_t parse, void *userdata, ProcCmdlineFlags flags);