ostree admin instutil set-kargs: make more flexible

ostree admin instutil set-kargs: make more flexible

Add command line arguments:
 --import-proc-cmdline: import values from /proc/cmdline
 --merge: import current values
 --replace=ARG=VALUE: replace value
 --append=ARG=VALUE: append a new argument

Extra command line arguments are treated like --append=, which
gives backwards compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=731051