Implement support of keeping readall capabilities after UID/GID switch

Implement support of keeping readall capabilities after UID/GID switch

Extend drop() function interface to accept 3rd parameter specifying
whether process should keep capabilities required to read and search
files and directories regardless of their access permissions.

Introduce new bacula-fd option (-k) specifying that readall capabilities
should be kept after UID/GID switch.

The change moves drop() definition from bsys.c to new file priv.c,
which is necessary to avoid linking every bacula executable with -lcap.
If drop() would remain in bsys.c, then every executable which directly
or indirectly uses other functions defined in bsys.c would have to be
linked with -lcap, unless libbac is built as a shared library.

While the change itself is portable, the implementation is Linux
specific, it uses libcap to keep CAP_DAC_READ_SEARCH capability.
If libcap is not available, or OS does not have sys/prctl.h,
sys/capability.h, prctl(2), setreuid(2) and PR_SET_KEEPCAPS, then
this change is almost noop.