HTTPDownload: bound libcurl connect-phase via GetNativeHandle() - #565

…mule-project#565)

Fix the CMake wiring so mmap is actually reachable (amule-project#34): the port never
set HAVE_MMAP (glib21.cmake probed munmap, not mmap) and never passed
ENABLE_MMAP to the compiler, so USE_MMAP was dead on every CMake build even
with -DENABLE_MMAP=YES. Add the missing mmap/sigaction probes and a single
MMAP_SUPPORTED capability macro (set when mmap/munmap/sysconf/_SC_PAGESIZE/
sigaction are all present) gating the code path, the preferences checkbox and
the EC tag. ENABLE_MMAP becomes a default-ON build opt-out (no external
dependency; all libc).

Make mmap use a runtime preference (MMapEnabled, default OFF):
- CFileArea reads an atomic flag at ReadAt/StartWriteAt; the per-area mode is
  captured in m_mmap_buffer so FlushAt/Close stay state-driven -- safe to flip
  live with active downloads and uploads (in-flight blocks finish in their
  original mode).
- The SIGSEGV/SIGBUS recovery handler installs once, when mmap is first
  enabled, rather than per read/write and never when off, so it cannot collide
  with sanitizers/debuggers/crash reporters.
- Advanced-tab checkbox; capability negotiated over EC (EC_TAG_FILES_MMAP_
  SUPPORTED/_ENABLED) so amulegui/amulecmd/amuleweb/amuleapi show the toggle
  only when the connected daemon supports mmap (tag-presence == capability;
  works cross-platform, e.g. Windows GUI to a Linux daemon).

Relax the 4 GiB per-area cap (forum 16444): it only guards a 32-bit off_t.
On 64-bit off_t -- every modern LFS build, including 32-bit-with-LFS (aMule
inherits _FILE_OFFSET_BITS=64 from wx) -- mmap handles large offsets natively,
verified against a >4 GiB region, so large files get mmap for their whole
length; the cap remains only for a (wx-precluded) non-LFS 32-bit build.

…-project#566)

Follow-up to amule-project#565: surface the memory-mapped file I/O preference on the REST
API. files.mmap_supported is a read-only daemon capability (mirrors
upnp_available), read from the EC_TAG_FILES_MMAP_SUPPORTED tag; files.mmap_enabled
is the runtime value. A PATCH that sets mmap_enabled is rejected with 409 when the
connected daemon lacks mmap support, so the option is only writable against a core
that can actually use it.

Updates docs/api/REFERENCE.md and the 15-preferences-patch curl smoke (round-trip
on a mmap-capable daemon; the 409 capability gate otherwise).