stat: Implement stat.h definitions

Created by: sajattack

I'm gonna work on this header, just making an issue so everyone knows and we don't duplicate work.

• chmod
• fchmod
• fstat
• lstat
• mkdir
• mkfifo
• mknod (POSIX only needs FIFO support)
• stat
• umask

More from the latest spec (https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_stat.h.html):

• fchmodat
• fstatat (!695 (merged))
• mkdirat
• mkfifoat
• utimensat