basic/cap-list: parse/print numerical capabilities

We would refuse to print capabilities which were didn't have a name
for. The kernel adds new capabilities from time to time, most recently
cap_bpf. 'systmectl show -p CapabilityBoundingSet ...' would fail with
"Failed to parse bus message: Invalid argument" because
capability_set_to_string_alloc() would fail with -EINVAL. So let's
print such capabilities in hexadecimal:

CapabilityBoundingSet=cap_chown cap_dac_override cap_dac_read_search
  cap_fowner cap_fsetid cap_kill cap_setgid cap_setuid cap_setpcap
  cap_linux_immutable cap_net_bind_service cap_net_broadcast cap_net_admin
  cap_net_raw cap_ipc_lock cap_ipc_owner 0x10 0x11 0x12 0x13 0x14 0x15 0x16
  0x17 0x18 0x19 0x1a ...

For symmetry, also allow capabilities that we don't know to be specified.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1853736.