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 redox-os/relibc!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