SH: use is_whitespace()

SH: use is_whitespace()

Wherever blanks are permissible in input, tabs ought to be permissible,
too. This is particularly relevant when -f is passed to gas (alongside
appropriate input). Also convert open-coded checks as well as an
ISSPACE() use. At the same time use is_end_of_stmt() instead of
(kind-of-)open-coded checks in adjacent code.