This will allow us to redefine the exit function.

For example:
```
#define exit(code) { \
    fprintf(stderr, "%s:%d: exit(%s) in function ‘%s’\n",
        __FILE__, __LINE__, #code, __func__); \
    _exit(code); \
}
```

General test cleanups

See merge request redox-os/relibc!193

unistd: Generate correct C defines for PATH_SEPARATOR

See merge request !194

When the execution reaches the end of the main functions, they implicitly return a successful status.

tests: set C11, enable pedantic warnings, fix GCC and Clang warnings

See merge request redox-os/relibc!192

ctype: Implement _tolower, _toupper

See merge request redox-os/relibc!191

ctype: Add tests for toascii, tolower, toupper

See merge request redox-os/relibc!190

ctype: Fix iscntrl, isgraph, ispunct, make tests exhaustive

See merge request !189

iscntrl: Didn't handle the EOF macro correctly
isgraph: Didn't handle '~' (tilde) correctly
ispunct: Several issues

Implement ttyname by adding fpath function to Sys. Remove realpath function and use fpath internally