Unix rtlib: Make __fb_bg_mutex recursive like the others

This should help prevent dead-locks in case a signal handler from
src/rtlib/signals.c will be called (e.g. in case of SIGINT), because it
calls fb_End() -> fb_hEnd() -> fb_hExitConsole() which may attempt to
acquire the __fb_bg_mutex recursively. Of course this is not a full fix
for the broken signal handler (it should not invoke any non-async-signal-
safe functions).
Unix rtlib: Avoid unnecessary terminal size query during fb_hExitConsole()

Now an FB program that doesn't use console commands can run without ever
having to query the terminal size. This also helps with the unsafe signal
handler issue (bug #740).
Unix rtlib: Fix potential double cursor pos query in fb_ConsoleGetXY()

fb_ConsoleGetXY() could query the cursor position twice (once during the
fb_hRecheckConsoleSize(), and once manually), which is unnecessary.
This belongs to commit 89a57cea and perhaps also commit 813d0d01.
rtlib: Remove some unnecessary FB_HANDLE_USED checks

fb_FilePutDataEx() already does it.
inc: Update fmod.bi, old FMOD 3.75