interp: handle select clause

the parser already supports the select command but the interpreter treats it just like any other for command

given the syntax:

select name [in words ...]; do commands; done

fix support by:

expanding and printing the words following in, to generate a list of items, each preceeded by a number and a close parenthesis
display the prompt and read the line from stdin
assigning the selected option to a special variable in Bash called REPLY
support PS3, another special variable in Bash, which can be set to the prompt while the shell awaits for input. if not provided, defaults to #?:

additionally, select should loop until an input is provided. in case of an invalid input, the name will be empty

$ bash -c 'PS3="pick one: "; select SEL in foo; do echo $REPLY; break; done'
1) foo
pick one:
1) foo
pick one:
1) foo
pick one: 1

mvdan / sh