Install: correctly report symlink creations

Install: correctly report symlink creations

All callers of create_symlink(), such as install_info_symlink_wants(), expect
that to return > 0 if it actually did something, and then return that number.
unit_file_enable() uses that to determine if any action was done
(carries_install_info != 0) and if not, show a "The unit files have no
[Install] section" warning.

Return 1 instead of 0 in the two code paths of create_symlink() when the link
was created or replaced with a new value.

This fixes getting a bogus "No [Install] section" warning when enabling a unit
with full path, like "systemctl enable /some/path/myunit.service".