GOption: don't use "rand" in code example

rand() is a function defined in the libc, so our code example gives
warnings if you try to compile it.  Fix that.

https://bugzilla.gnome.org/show_bug.cgi?id=722025
g_option_context_parse_strv: use UTF-8 on Windows

Add another difference to the freshly-added g_option_context_parse_strv:
now, on Windows, its arguments on to be in UTF-8 instead of the argv[]
encoding (from the system codepage).

The documentation teases g_win32_get_command_line() which is a new
GLib-filename-encoding-based function that will be added in a following
commit.

https://bugzilla.gnome.org/show_bug.cgi?id=722025
win32: add g_win32_get_command_line()

This returns the command line in GLib filename encoding format (ie:
UTF-8) for use with g_option_context_parse_strv().

This will allow parsing of Unicode commandline arguments on Windows,
even if the characters in those arguments fall outside of the range of
the system codepage.

https://bugzilla.gnome.org/show_bug.cgi?id=722025
g_file_new_for_commandline_arg: clarify encoding

Add a note to the documentation for g_file_new_for_commandline_arg()
that this function is intended to operate on strings already in the GLib
filename encoding on Windows.

This has been the case for a long time, but this documents the
requirement.

https://bugzilla.gnome.org/show_bug.cgi?id=722025
GApplication: change commandline encoding policy

Clarify in the documentation that the commandline arguments passed
around by GApplication (to local_command_line and returned via
g_application_command_line_get_arguments()) are in the GLib filename
encoding (ie: UTF-8) on Windows, not the system code page.

Fix the mismatch that would result from having argv passed to
g_application_run() in main() on Windows (where it is in the system
code page) by ignoring argc/argv on Windows and calling
g_win32_get_command_line() for ourselves.  Document this.

This might be a slight API break on Windows: we documented that it was
possible to call g_application_run() with arguments other than argc/argv
and now doing that will result in those arguments being ignored.  It has
always been recommended practice to only call g_application_run() from
main() directly, however, and all of our code examples have shown only
this.  We will see if this causes any issues and consider reevaluating
the situation if so.

https://bugzilla.gnome.org/show_bug.cgi?id=722025
GOptionContext: add some notes about encodings

Add a note to the overview documentation for GOptionContext about why
you need to be careful about argv encoding on UNIX and about why you
should avoid argv entirely on Windows.  Mention some possible
alternative approaches, including a code example.

https://bugzilla.gnome.org/show_bug.cgi?id=722025