Complete capabilites with and without "CAP_" prefix.

[albers]

This extends bash completion for the capabilities after --cap-add and --cap-drop to also include the CAP_ prefixed variants.
See #2687 (comment).

Before:

$ docker run --cap-drop <TAB TAB>
ALL               DAC_OVERRIDE      KILL              NET_RAW           SETPCAP
AUDIT_WRITE       FOWNER            MKNOD             SETFCAP           SETUID
CHOWN             FSETID            NET_BIND_SERVICE  SETGID            SYS_CHROOT

Now:

$ docker run --cap-drop <TAB TAB>
ALL                   CAP_FSETID            CAP_SETGID            FOWNER                SETFCAP
AUDIT_WRITE           CAP_KILL              CAP_SETPCAP           FSETID                SETGID
CAP_AUDIT_WRITE       CAP_MKNOD             CAP_SETUID            KILL                  SETPCAP
CAP_CHOWN             CAP_NET_BIND_SERVICE  CAP_SYS_CHROOT        MKNOD                 SETUID
CAP_DAC_OVERRIDE      CAP_NET_RAW           CHOWN                 NET_BIND_SERVICE      SYS_CHROOT
CAP_FOWNER            CAP_SETFCAP           DAC_OVERRIDE          NET_RAW

Note the special treatment of ALL: CAP_ALL is not added.

This makes completion consistent with the completion of signals, e.g.

$ docker run --stop-signal <TAB TAB>
CONT     INT      QUIT     SIGHUP   SIGKILL  SIGSTOP  SIGUSR1  STOP     USR1
HUP      KILL     SIGCONT  SIGINT   SIGQUIT  SIGTERM  SIGUSR2  TERM     USR2

[silvin-lubecki]

LGTM

[thaJeztah]

LGTM, thanks!