libsmartcols: sanitize variable names on export output

libsmartcols: sanitize variable names on export output

The shells are very restrictive about variable names, only [:alnum:]
chars are allowed (and alphabetic chars as the first char).  The
library will replace "bad" chars with "_". The char '%' at the end is
replaced by _PCT.

Addresses: https://github.com/karelzak/util-linux/issues/1201
Signed-off-by: Karel Zak <kzak@redhat.com>