terminal-util: introduce terminal_{new,detach}_session helpers

Prompted by #35761

src/basic/terminal-util.c

@@ -498,6 +498,22 @@ int release_terminal(void) {
         return r;
 }
 
+int terminal_new_session(void) {
+
+        /* Make us the new session leader, and set stdin tty to be our controlling terminal.
+         *
+         * Why stdin? Well, the ctty logic is relevant for signal delivery mostly, i.e. if people hit C-c
+         * or the line is hung up. Such events are basically just a form of input, via a side channel
+         * (that side channel being signal delivery, i.e. SIGINT, SIGHUP et al). Hence we focus on input,
+         * not output here. */
+
+        if (!isatty_safe(STDIN_FILENO))
+                return -ENXIO;
+
+        (void) setsid();
+        return RET_NERRNO(ioctl(STDIN_FILENO, TIOCSCTTY, 0));
+}
+
 int terminal_vhangup_fd(int fd) {
         assert(fd >= 0);
         return RET_NERRNO(ioctl(fd, TIOCVHANGUP));

src/basic/terminal-util.h

@@ -7,6 +7,7 @@
 #include <syslog.h>
 #include <sys/types.h>
 #include <termios.h>
+#include <unistd.h>
 
 #include "macro.h"
 #include "time-util.h"
@@ -64,6 +65,12 @@ typedef enum AcquireTerminalFlags {
 int acquire_terminal(const char *name, AcquireTerminalFlags flags, usec_t timeout);
 int release_terminal(void);
 
+int terminal_new_session(void);
+static inline void terminal_detach_session(void) {
+        (void) setsid();
+        (void) release_terminal();
+}
+
 int terminal_vhangup_fd(int fd);
 int terminal_vhangup(const char *tty);