gril: fix GRilIO read under boundary conditions

Close: #142

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gril: remove ril_s->read_so_far

struct ril_s::read_so_far is never assigned a non-zero value outside
the while-loop in function `new_bytes` in gril/gril.c.

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gril: dispatch multiple parcels in burst

Close: #141

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>