fix incorrect sequence generation in *rand48 prng functions

patch by Jens Gustedt. this fixes a bug reported by Nadav Har'El. the
underlying issue was that a left-shift by 16 bits after promotion of
unsigned short to int caused integer overflow. while some compilers
define this overflow case as "shifting into the sign bit", doing so
doesn't help; the sign bit then gets extended through the upper bits
in subsequent arithmetic as unsigned long long. this patch imposes a
promotion to unsigned prior to the shift, so that the result is
well-defined and matches the specified behavior.

include cleanups: remove unused headers and add feature test macros

prng: make rand_r have 2^32 period instead of 2^31

this is a minor fix to increase the period of the obsolete rand_r a bit.
an include header in __rand48_step.c is fixed as well.

initial check-in, version 0.5.0