libsmartcols: clean up flags usage

 * rename flags functions to scols_table_enable_*
 * rename *_no_foo() functions to _nofoo()
 * output formats are mutually exclusive, so don't use flags there
 * don't assume symbols in scols_new_table(), use scols_table_set_symbols()

Signed-off-by: Karel Zak <kzak@redhat.com>