dc(1): Sync with OpenBSD

dc(1): Sync with OpenBSD

Changes include :

 * show an error message if the argument is a dir

 * check value against UINT_MAX before casting to u_int; while
   there fix a buglet in bexp(): x * UINT_MAX only overflows if x > 0

 * fix fractional number exponentiation

 * use BN_set_negative() and BN_is_negative() instead of subtracting or
   comparing to zero

 * For exponenttion, only warn if the fractional part of the exponent
   is non-zero and avoid div by zero.

 * let length(0.000) conform to what gnu bc does

 * check for unitialized elements when accessing an array

 * although there's no EOF condition defined for the handling of '?',
   but anyway reset the eof condition on stdin, so you don't get an
   infinite loop in the typical program after a ^D.

 * convert malloc to calloc

 * fix three leaks

 * use size_t as stack size and ssize_t as stack pointer

 * add missing bn_check

Revision for updated OpenBSD files are :

 - bcode.c : 1.45
 - bcode.h : 1.7
 - dc.c : 1.11
 - inout.c : 1.17
 - stack.c : 1.11

Obtained-from: OpenBSD