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/fcntl.h

@@ -6,10 +6,22 @@
 #define O_RDONLY 0x0000
 #define O_WRONLY 0x0001
 #define O_RDWR   0x0002
+#define O_ACCMODE 0x0003
 #define O_CREAT  0x0040
+#define O_EXCL   0x0080
 #define O_TRUNC  0x0200
 #define O_APPEND 0x0400
+#define O_NONBLOCK 0x0800
+
+#define F_GETFL 3
+#define F_SETFL 4
+
+#define FD_CLOEXEC 1
 
 int open(const char *pathname, int flags, ...);
+int fcntl(int fd, int cmd, ...);
+int dup(int oldfd);
+int dup2(int oldfd, int newfd);
+int pipe(int pipefd[2]);
 
 #endif