Fix various implicit conversions from size_t to smaller types

Basically various trivial instances of the following MSVC compiler warning:

../gio/gio-tool-set.c(50): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data

Plus a few commits with a couple of slightly-more-involved changes along the same lines. See the commit messages for details.

Signed-off-by: Philip Withnall pwithnall@gnome.org