fix(upnp): pin LC_CTYPE = C around ASCII tolower() in UPnPBase - #208

UPnPBase.cpp has four call sites that rely on tolower() producing
ASCII semantics:

  * stdStringIsEqualCI / stdStringStartsWithCI -- the two
    case-insensitive std::string helpers used throughout the file.
  * CUPnPService::Execute -- direct comparison against 'i' and 'n' to
    validate the UPnP argument direction string ("in" / "out").
  * CUPnPControlPoint::Callback BYEBYE case -- transforms the
    discovered device-type URN to lowercase before matching against
    UPnP::Device::IGW.

Under tr_TR.UTF-8 (and any other Turkish-locale variant), libc
tolower('I') returns U+0131 (dotless i) instead of 'i'. This
silently breaks UPnP direction validation, device-type matching,
namespace lowercase comparisons, and HTTP-header lowercasing --
amuled's UPnP auto-portforward stops working for Turkish-locale
users.

Before amule-project#205 (i18n: fix non-ASCII mangling under default C locale),
amuled stayed on the C locale throughout its lifetime and these
sites were silently safe. The fix in amule-project#205 made amuled pick LC_CTYPE
up from the environment so unicode2char() emits real UTF-8 -- which
exposed the latent Turkish-i issue documented above.

Wrap each tolower() scope in CCtypeAsciiScope -- the project's
existing RAII helper that pins LC_CTYPE = "C" for the duration of
its scope and restores it on destruction. Same pattern already used
in CamuleFileConfig.h (amule-project#852) and MaxMindDBDatabase.cpp.

amule (GUI) was already exposed to this bug via wxLocale's
setlocale(LC_ALL) on Turkish-language installs; the wrap fixes it
there too as a free side effect.