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
ostree admin: Add a --print-current-dir option

Add an option --print-current-dir that prints the current deployment
directory to stdout and exits.

https://bugzilla.gnome.org/show_bug.cgi?id=731051
Add test case for 'admin instutil set-kargs'

Test out the newly added options to 'instutil set-kargs' along with
the existing functionality.

https://bugzilla.gnome.org/show_bug.cgi?id=731051
Test 'ostree admin --print-current-dir'

Add a test for the --print-current-dir option

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