Fix dropping of all capabilities

Fix dropping of all capabilities

From fd.o bug 88898:

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.

Based-on-a-patch-by: mustrumr97@gmail.com
https://bugs.freedesktop.org/show_bug.cgi?id=88898

We must be careful not to leave PR_SET_KEEPCAPS on. We could use the
setresuid() call to drop capabilities, but the rules when capabilities
are dropped are fairly complex, since a transition to non-zero uid must
happen. Let's instead keep the capabilities during setresuid(), and drop
them later.