util-lib: various improvements to kernel command line parsing

This improves kernel command line parsing in a number of ways:

a) An kernel option "foo_bar=xyz" is now considered equivalent to
   "foo-bar-xyz", i.e. when comparing kernel command line option names "-" and
   "_" are now considered equivalent (this only applies to the option names
   though, not the option values!). Most of our kernel options used "-" as word
   separator in kernel command line options so far, but some used "_". With
   this change, which was a source of confusion for users (well, at least of
   one user: myself, I just couldn't remember that it's systemd.debug-shell,
   not systemd.debug_shell). Considering both as equivalent is inspired how
   modern kernel module loading normalizes all kernel module names to use
   underscores now too.

b) All options previously using a dash for separating words in kernel command
   line options now use an underscore instead, in all documentation and in
   code. Since a) has been implemented this should not create any compatibility
   problems, but normalizes our documentation and our code.

c) All kernel command line options which take booleans (or are boolean-like)
   have been reworked so that "foobar" (without argument) is now equivalent to
   "foobar=1" (but not "foobar=0"), thus normalizing the handling of our
   boolean arguments. Specifically this means systemd.debug-shell and
   systemd_debug_shell=1 are now entirely equivalent.

d) All kernel command line options which take an argument, and where no
   argument is specified will now result in a log message. e.g. passing just
   "systemd.unit" will no result in a complain that it needs an argument. This
   is implemented in the proc_cmdline_missing_value() function.

e) There's now a call proc_cmdline_get_bool() similar to proc_cmdline_get_key()
   that parses booleans (following the logic explained in c).

f) The proc_cmdline_parse() call's boolean argument has been replaced by a new
   flags argument that takes a common set of bits with proc_cmdline_get_key().

g) All kernel command line APIs now begin with the same "proc_cmdline_" prefix.

h) There are now tests for much of this. Yay!

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