Support empty properties in INI files
    
    The ini file parser doesn't support parsing properties with no value.
    This patch adds that feature, as it turned out to be a pre-requisite
    for reading Linux Kernel ABI whitelist files.
    
    	* include/abg-ini.h (simple_property::simple_property): Add a new
    	constructor for empty values.
    	(simple_property::has_empty_value): Declare new member function.
    	* src/abg-ini.cc (simple_property::{simple_property,
    	has_empty_value}): Define new member functions.
    	(read_context::read_property): Support reading a property with no
    	value.
    	(write_property_value, write_property): Support writting a
    	property with empty value.
    
    Signed-off-by: Dodji Seketeli <dodji@redhat.com>