libc: Add program_invocation_name

This is a GNU extension, but I'm fine with adding it to libc.
It's guarded by the _GNU_SOURCE feature test macro anyways.

libs/libc/include/errno.h

@@ -28,4 +28,9 @@ extern int errno;
 #define ENOTSUP 95    // Operation not supported
 #define EOPNOTSUPP 95 // Operation not supported
 
+#ifdef _GNU_SOURCE // Give it only to programs that ask for it.
+/* Name used to invoke calling program. Same value as argv[0] in main(), but can be used globally. */
+extern char* program_invocation_name;
+#endif
+
 #endif

libs/libc/src/errno.cpp

@@ -1,3 +1,4 @@
 #include <errno.h>
 
-int errno;
+int errno;
+char* program_invocation_name;

libs/libc/src/init.cpp

@@ -51,12 +51,17 @@ static void check_for_file(int fd, FILE** target_stream, const char* path, const
     else { *target_stream = fdopen(fd, mode); }
 }
 
-extern "C" void initialize_libc()
+extern char* program_invocation_name;
+
+extern "C" void initialize_libc(int argc, char** argv)
 {
     check_for_file(STDIN_FILENO, &stdin, "/dev/kbd", "r");
     check_for_file(STDOUT_FILENO, &stdout, "/dev/console", "rw");
     check_for_file(STDERR_FILENO, &stderr, "/dev/console", "rw");
 
+    program_invocation_name =
+        argv[0]; // argv[0] should only be nullptr when being init, which shouldn't use program_invocation_name anyways.
+
     initialize_random();
     atexit(terminate_libc);
 }