vendor/bc: imoport release 5.0.0

This is a new major release with a number of changes and extensions:

• Limited the number of temporary numbers and made the space for them static so that allocating more space for them cannot fail.
• Allowed integers with non-zero scale to be used with power, places, and shift operators.
• Added greatest common divisor and least common multiple to lib2.bc.
• Made bc and dc UTF-8 capable.
• Added the ability for users to have bc and dc quit on SIGINT.
• Added the ability for users to disable prompt and TTY mode by environment variables.
• Added the ability for users to redefine keywords.
• Added dc's modular exponentiation and divmod to bc.
• Added the ability to assign strings to variables and array elements and pass them to functions in bc.
• Added dc's asciify command and stream printing to bc.
• Added bitwise and, or, xor, left shift, right shift, reverse, left rotate, right rotate, and mod functions to lib2.bc.
• Added the functions s2u(x) and s2un(x,n), to lib2.bc.