Make fclose/pclose distinct.

I failed to convince the compiler/linker to just refrain (via factoring out,
attribute `noinline`, or meddling with `--icf=none`), but luckily I noticed
that we should have CHECK_FP in each function for a better error message,
and the distinct error messages keep the two functions apart.

(Also add a missing CHECK_FP to `clearerr_unlocked`.)

Bug: http://b/116969207
Test: manual with a modified `crasher`
Change-Id: Ic122e90e94f7e22f486be57d3dac7137a225d682