core: resolve specifier in config_parse_exec()

core: resolve specifier in config_parse_exec()

When parse ExecXXX=, specifiers are not resolved in
config_parse_exec(). Finally, the specifiers are set into unit
properties. So, systemctl shows not resolved speicifier on "Process:"
field.
To set the exec properties well, resolve specifiers before parse the
rvale by unit_full_printf();