contrib/bc: merge from vendor release 6.2.2

This update fixes a few issues in history editing and the processing
of the "quit" function. The "quit" function will no longer cause bc
to exit when encountered in a script file (before any command from
the script has been executed).

New functions is_number(), is_string return 1 if the passed argument
is a number resp. a string. The asciify() function has been extended
to support the conversion of an array of numbers into a string.

Merge commit '1a63323d17fedb05b6962853e821c9d7c6b9853e'