MapWindow/StencilMapCanvas: Return bool in Commit() as indication if something was rendered
AirspaceRenderer: Optimize airspace fill rendering

Don't render airspace fill cache if no filled airspace is relevant
for the displayed map area.
OS/SocketDescriptor: don't use accept4() on the Kobo

The Kobo's stock kernel is too old for accept4() with SOCK_CLOEXEC.
Fixes TRAC #3083.
Makefile: libpng always depends on zlib

Remove the condition that checks TARGET_STATIC, this was bogus.  Fixes
building a dynamic executable with a static libpng.a.
Makefile: enable dynamic linking on the Kobo

Turns out that dynamic linking works pretty well on the Kobo with its
stock libraries (glibc, libstdc++).

The binaries are not smaller, and the NSS modules do not crash XCSoar
anymore (TRAC #3078).
Language: enable built-in MO files on Mac OS X

The standard gettext interface doesn't seem to work well on Mac OS X,
or we havn't figured out how to do it properly (TRAC #2558).  Since
nobody is currently maintaining the OS X port, I'm doing it the easy
way by moving the MO files into the XCSoar binary.
Engine/Airspace/AirspaceWarningManger: Prevent creation of temporary warnings

The AirspaceWarningManger used many temporary warning objects which
were immediately discarded when the warning later turned out to be
not relevant causing a constantly incremented serial number.

This had the side effect of causing invalidation of the
cached airspace fill patterns used in the AirspaceRender since
it relied on the serial to trigger cache updates.
po: update French translation
Kobo/WPASupplicant: handle recv()==EAGAIN

Fixes out-of-sync wpa_supplicant responses because the first few
receive attempts time out (TRAC #3091).
Event/Linux/TTYKeyboard: add API documentation
Event/Linux/TTYKeyboard: revert to NONE on malformed input
Event/Linux/TTYKeyboard: implement console codes for F1..F5

Implements TRAC #3086.
Kobo/WifiDialog: add "pure" attribute
Kobo/WifiDialog: move code to FindVisibleBySSID()
Kobo/WifiDialog: reset the "old" flag if FindVisibleBySSID() was successful
Kobo/WifiDialog: move code to Append()
Kobo/WifiDialog: move code to Find(WifiConfiguredNetworkInfo)
Kobo/WifiDialog: reset the "old" flag on new items
Kobo/WifiDialog: split the "old" flag

Postpone resetting "signal_level" and "id" to SweepList().  This
preserves the "id" attribute while we're updating the list.
Kobo/WifiDialog: find configured networks by id