Don't use the reserved keyword 'interface'

Don't use the reserved keyword 'interface'

On Win32 environments, the keyword 'interface' is a reserved keyword. Up
until now a hack was applied in soup-portability.h to '#undef interface'
so that various pieces of libsoup could use that keyword. However, due
to a recent change in the mingw-w64 toolchain this #undef causes more
breakage while #include'ing other Win32 headers.

As the keyword 'interface' is a reserved keyword on Win32, applications
or libraries shouldn't be messing around with that. This patch changes
various parts of libsoup where the keyword 'interface' is used as name
for variables. These variables have been renamed to 'iface'. Due to this
rename, the #undef hack can be dropped and libsoup can be compiled
cleanly against the latest mingw-w64 toolchain

https://bugzilla.gnome.org/show_bug.cgi?id=656402