lib: add frr_elevate_privs() wrapper

lib: add frr_elevate_privs() wrapper

Used as:
  frr_elevate_privs(&my_privs) {
    ... code ...
  }
and handles privilege raise/lower automatically in conjunction with the
C expression block.  This makes it impossible to accidentally exit a
function with privileges raised (and then running a whole bunch of other
code with privs.)

Signed-off-by: David Lamparter <equinox@diac24.net>