conf-parser: Set EXTRACT_RETAIN_ESCAPE when extracting words (#2917)

conf-parser: Set EXTRACT_RETAIN_ESCAPE when extracting words (#2917)

If you reference another unit with an escaped name, the escaped characters
should remain in the extracted word. This used to work correctly prior to
commit 34f253f0.

The problem can be seen when units with escaped names are referenced.

$ cat "/usr/lib/elogind/system/dev-disk-by\x2dlabel-eos\x2dswap.swap"
[Swap]
What=/dev/disk/by-label/eos-swap

[Install]
WantedBy=dev-disk-by\x2dlabel-eos\x2dswap.device

$ systemctl enable "dev-disk-by\x2dlabel-eos\x2dswap.swap"
Created symlink
/etc/elogind/system/dev-disk-byx2dlabel-eosx2dswap.device.wants/dev-disk-by\x2dlabel-eos\x2dswap.swap,
pointing to /usr/lib/elogind/system/dev-disk-by\x2dlabel-eos\x2dswap.swap.

The wants directory should be created with the x2ds escaped with \.