systemd-resolved fails to start:
Failed to drop capabilities: Operation not permitted

Broken in f11943c53ec181829a821c6b27acf828bab71caa.

Drop all capabilities:
1. prctl(PR_SET_KEEPCAPS, keep_capabilities != 0) // 0 when we drop all capabilities
2. setresuid() // bye bye capabilities
3. Add CAP_SETPCAP // fails because we have no capabilities
4. Reduce capability bounding set
5. Drop capabilities
6. prctl(PR_SET_KEEPCAPS, 0)

Capabilites should always be kept after setresuid() so that the capability bounding set can be reduced.

--- a/src/shared/capability.c
+++ b/src/shared/capability.c
@@ -249,7 +249,7 @@ int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
                 return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
 
         /* Ensure we keep the permitted caps across the setresuid(), if we need them */
-        if (prctl(PR_SET_KEEPCAPS, keep_capabilities != 0) < 0)
+        if (prctl(PR_SET_KEEPCAPS, 1) < 0)
                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 
         r = setresuid(uid, uid, uid);