Use Cell representation for errno

src/header/arpa_inet/mod.rs

src/header/dirent/mod.rs

src/header/errno/mod.rs

src/header/grp/mod.rs

src/header/netdb/mod.rs

src/header/pty/mod.rs

src/header/pwd/mod.rs

src/header/signal/mod.rs

src/header/stdio/getdelim.rs

src/header/stdio/helpers.rs

src/header/stdio/mod.rs

src/header/stdio/printf.rs

src/header/stdlib/mod.rs

src/header/string/mod.rs

src/header/sys_ioctl/redox.rs

src/header/sys_select/mod.rs

src/header/sys_uio/mod.rs

src/header/termios/mod.rs

src/header/time/mod.rs

src/header/unistd/brk.rs