Removed all function pointers in FILE, moved internal functions to be member functions of FILE, made relevant *mut FILEs into &mut FILE and made suitable functions safe again

Issue #89: Fix return types on string functions

* fix return types
* fix type casts on returns

Changed object type of function pointers from Option<*const (Fn(...))> to Option<fn(...)> for readability

wait and waitpid

Implement strstr() and strpbrk() from string.h

Update cbindgen and fix issues with signal

The sigaction struct should not be a typedef, but the sigset_t struct
should be a typedef.

Update cbindgen to parse modules with a path attribute and revert the
following commits:
 - 996fad70.
 - cfe98ab3.

Actual working tests on strspn and strcspn

Yet another fix for strcspn and strspn.