A few minor fixes after #4847 (#4850)

* Add a toggle to disable the C blitter in the Dev menu (depends on https://github.com/koreader/koreader-base/pull/882) (never shown if the JIT is disabled, grayed out if the C blitter is not installed)
* Fix a few sizeUtf8Text call sites that were doing a nil check in order to account for the new return type.
* Tweak statusbar handling to avoid spurious sizeUtf8Text warnings when it's hidden, and unify its behavior between being hidden via toggle, and hidden on book open (at least when all-at-once is not enabled).
* c.f., https://github.com/koreader/koreader-base/pull/882 (Android, PB, RGB32 & Legacy Kindle regression fixes).

setupkoenv.lua

@@ -8,13 +8,10 @@ package.cpath =
 
 -- set search path for 'ffi.load()'
 local ffi = require("ffi")
-ffi.cdef[[
-    char *getenv(const char *name);
-    int putenv(const char *envvar);
-    int _putenv(const char *envvar);
-]]
+local dummy = require("ffi/posix_h")
+local C = ffi.C
 if ffi.os == "Windows" then
-    ffi.C._putenv("PATH=libs;common;")
+    C._putenv("PATH=libs;common;")
 end
 local ffi_load = ffi.load
 -- patch ffi.load for thirdparty luajit libraries