Kernel/LibC: Add initial pseudo terminal support

This patch implements posix_openpt() and ptsname()

grantpt() and unlockpt() are left in LibC as stubs, as posix_openpt
currently does all of the needed work.

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

@@ -88,6 +88,8 @@ __BEGIN_DECLS
 	O(SYS_SIGPENDING,		sigpending)		\
 	O(SYS_SIGPROCMASK,		sigprocmask)	\
 	O(SYS_SETITIMER,		setitimer)		\
+	O(SYS_POSIX_OPENPT,		posix_openpt)	\
+	O(SYS_PTSNAME,			ptsname)		\
 
 enum Syscall
 {

userspace/libraries/LibC/stdlib.cpp

@@ -519,6 +519,31 @@ char* mktemp(char*)
 	ASSERT_NOT_REACHED();
 }
 
+int posix_openpt(int oflag)
+{
+	return syscall(SYS_POSIX_OPENPT, oflag);
+}
+
+int grantpt(int)
+{
+	// currently posix_openpt() does this
+	return 0;
+}
+
+int unlockpt(int)
+{
+	// currently posix_openpt() does this
+	return 0;
+}
+
+char* ptsname(int fildes)
+{
+	static char buffer[PATH_MAX];
+	if (syscall(SYS_PTSNAME, fildes, buffer, sizeof(buffer)) == -1)
+		return nullptr;
+	return buffer;
+}
+
 size_t mbstowcs(wchar_t* __restrict pwcs, const char* __restrict s, size_t n)
 {
 	auto* us = reinterpret_cast<const unsigned char*>(s);