libpari precision handling changes

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear PARI developers,

I committed a change to the way the precision is manipulated in the 
C library.

Now the precision is a multiple of BITS_IN_LONG.
DEFAULTPREC is 64      (instead of 3)
MEDDEFAULTPREC is 128  (instead of 4)
BIGDEFAULTPREC is 192  (instead of 5)
realprec return a number of bits, cgetr takes a number of bits as input, the
prototype code 'p' of 'install' passes a number of bits so every function that
takes the argument prec, will receive a number of bits etc.

While this is a major change, this should not break most C programs that used
the libpari API correctly. For example, this does not impact GP2C-generated
code.

However this might affect programs written in other languages that they cannot
use the C headers.

This does not affect GP.

Cheers,
Bill.