math: bessel cleanup (j1.c and j1f.c)

a common code path in j1 and y1 was factored out so the resulting
object code is a bit smaller

unsigned int arithmetics is used for bit manipulation

j1(-inf) now returns 0 instead of -0

an incorrect threshold in the common code of j1f and y1f got fixed
(this caused spurious overflow and underflow exceptions)

the else branch in pone and pzero functions are fixed
(so code analyzers dont warn about uninitialized values)