pager: set PR_DEATHSIG for pager to SIGINT rather than SIGTERM

"less" doesn't properly reset its terminal on SIGTERM, it does so only
on SIGINT. Let's thus configure SIGINT instead of SIGTERM.

I think this is something less should fix too, and clean up things
correctly on SIGTERM, too. However, given that we explicitly enable
SIGINT behaviour by passing "K" to $LESS I figure it makes sense if we
also send SIGINT instead of SIGTERM to match it.

Fixes: #16084

src/basic/process-util.c

@@ -1301,8 +1301,8 @@ int safe_fork_full(
                                        r, "Failed to rename process, ignoring: %m");
         }
 
-        if (flags & FORK_DEATHSIG)
-                if (prctl(PR_SET_PDEATHSIG, SIGTERM) < 0) {
+        if (flags & (FORK_DEATHSIG|FORK_DEATHSIG_SIGINT))
+                if (prctl(PR_SET_PDEATHSIG, (flags & FORK_DEATHSIG_SIGINT) ? SIGINT : SIGTERM) < 0) {
                         log_full_errno(prio, errno, "Failed to set death signal: %m");
                         _exit(EXIT_FAILURE);
                 }