Improve sysconf.h on Linux

I copied the constants and definitions from musl which we can presume is
correct. Relibc is missing some features which affects our sysconf as
well, such as constants in limits.h or a working resources.h.

src/header/limits/mod.rs

@@ -4,3 +4,8 @@ pub const HOST_NAME_MAX: usize = 64;
 pub const PASS_MAX: usize = 128;
 pub const PATH_MAX: usize = 4096;
 pub const NGROUPS_MAX: usize = 65536;
+
+// TODO: 4096 for most architectures as determined by a quick grep of musl's source; need a better
+// way to determine it for other archs or to hard code a value.
+#[cfg(target_os = "linux")]
+pub const PAGE_SIZE: usize = 4096;