feat: Add signals, exec/wait, and FD/pipe support

Introduce process lifecycle and POSIX-like features: add parent_pid, pgid, exited/exit_status, signal state and handlers, waitpid/reap, and an exec-replace function. Refactor file descriptor handling to use fd_kind/fd_flags with reference-counted file refs and in-process pipes; implement open/read/write/close/seek/tell/size/dup/dup2/pipe/fcntl semantics and O_* flags. Add syscall handlers for exec, waitpid, kill/signal, sigaction, sigprocmask, sigpending, meminfo/ticks and map many SYSTEM_CMD_* constants; deliver signals from the syscall path. Cleanup/terminate logic updated to free resources correctly and initialize kernel/user processes with new state. Misc: minor syscall/table renames (wallpaper), helper utilities (process_close_fd_inner, process_init_signal_state) and paging/stack handling for exec.

src/userland/libc/errno.h

@@ -4,6 +4,7 @@
 extern int errno;
 
 #define EDOM 33
+#define E2BIG 7
 #define ERANGE 34
 #define EINVAL 22
 #define EISDIR 21
@@ -13,6 +14,14 @@ extern int errno;
 #define EBADF 9
 #define EIO 5
 #define EEXIST 17
+#define EAGAIN 11
+#define EINTR 4
+#define ECHILD 10
+#define ENOTSUP 95
+#define EPIPE 32
+#define ENOTDIR 20
+#define EBUSY 16
+#define ESPIPE 29
 #define ENOSYS 38
 
 #endif