libsmartcols: change "export" behavior, add "shellvar" flag

libsmartcols: change "export" behavior, add "shellvar" flag

In version 2.37 the "export" output format automatically modifies
column names to be compatible with shell's requirements for variable
names. This change was backwardly incompatible for people who use for
example "lsblk -P" in non-shell environment.

It's painful to modify this behavior again in v2.38, but it seems
better to add a way how to control this behavior independently on
output format as it seems attractive feature for more use-cases.

This commit introduces scols_table_enable_shellvar() function to
enable/disable this feature.

It also introduces

	scols_column_set_name
	scols_column_get_name
	scols_column_get_name_as_shellvar
	scols_table_is_shellvar

to make it easy to work with column names.

Fixes: https://github.com/util-linux/util-linux/issues/1594
Signed-off-by: Karel Zak <kzak@redhat.com>