GOptionContext: add memory-friendly parse mode

Add g_option_context_parse_strv() that obeys the normal memory conventions for
dealing with a strv instead of assuming that we're dealing with the 'argv'
parameter to main().

This will help for using GOptionContext with GApplication.

https://bugzilla.gnome.org/show_bug.cgi?id=721947
tests: fix leaks in option-context test

Use the new g_option_context_parse_strv() to patch up some leaks in some
insufficiently-argv-emulating testcases in option-context.c.

This gives some test coverage of the new function while also making
option-context now leak-free.

https://bugzilla.gnome.org/show_bug.cgi?id=721947