proc-cmdline: introduce proc_cmdline_strv()

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.