Kernel/LibC: Implement realpath

realpath is implemented as a syscall. This is not really required but it
was the easiest way to get it working as there is already path
canonicalization at kernel level.

userspace/libraries/LibC/include/sys/syscall.h

@@ -81,6 +81,7 @@ __BEGIN_DECLS
 	O(SYS_GETSOCKNAME,		getsockname)	\
 	O(SYS_GETSOCKOPT,		getsockopt)		\
 	O(SYS_SETSOCKOPT,		setsockopt)		\
+	O(SYS_REALPATH,			realpath)		\
 
 enum Syscall
 {

userspace/libraries/LibC/stdlib.cpp

@@ -7,6 +7,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <strings.h>
+#include <sys/syscall.h>
 #include <unistd.h>
 
 #include <icxxabi.h>
@@ -368,6 +369,22 @@ char* getenv(const char* name)
 	return nullptr;
 }
 
+char* realpath(const char* __restrict file_name, char* __restrict resolved_name)
+{
+	char buffer[PATH_MAX] {};
+	long canonical_length = syscall(SYS_REALPATH, file_name, buffer);
+	if (canonical_length == -1)
+		return NULL;
+	if (resolved_name == NULL)
+	{
+		resolved_name = static_cast<char*>(malloc(canonical_length + 1));
+		if (resolved_name == NULL)
+			return NULL;
+	}
+	strcpy(resolved_name, buffer);
+	return resolved_name;
+}
+
 int system(const char* command)
 {
 	// FIXME