exec: Add kill action to system call filters

Define explicit action "kill" for SystemCallErrorNumber=.

In addition to errno code, allow specifying "kill" as action for
SystemCallFilter=.

---
v7: seccomp_parse_errno_or_action() returns -EINVAL if !HAVE_SECCOMP
v6: use streq_ptr(), let errno_to_name() handle bad values, kill processes,
 init syscall_errno
v5: actually use seccomp_errno_or_action_to_string(), don't fail bus unit
parsing without seccomp
v4: fix build without seccomp
v3: drop log action
v2: action -> number

test/test-execute/exec-systemcallfilter-override-error-action.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=Test for SystemCallFilter with specific kill action overriding default errno action
+
+[Service]
+ExecStart=/usr/bin/python3 -c 'import os\ntry: os.uname()\nexcept Exception as e: exit(e.errno)'
+Type=oneshot
+SystemCallFilter=~uname:kill
+SystemCallErrorNumber=EILSEQ

test/test-execute/exec-systemcallfilter-override-error-action2.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=Test for SystemCallFilter with specific errno action overriding default kill action
+
+[Service]
+ExecStart=/usr/bin/python3 -c 'import os\ntry: os.uname()\nexcept Exception as e: exit(e.errno)'
+Type=oneshot
+SystemCallFilter=~uname:EILSEQ
+SystemCallErrorNumber=kill