Fix building on targets with unsigned c_char

src/header/monetary/strfmon.rs

src/header/netdb/host.rs

src/header/netdb/mod.rs

src/header/stdio/helpers.rs

src/header/stdio/mod.rs

src/header/stdlib/mod.rs

src/header/string/mod.rs

src/platform/redox/socket.rs

src/platform/types.rs