core/device: do not drop backslashes in SYSTEMD_WANTS=/SYSTEMD_USER_WANTS= (#35869)

Let consider the following udev rules:
```
PROGRAM="/usr/bin/systemd-escape foo-bar-baz", ENV{SYSTEMD_WANTS}+="test1@$result.service"
PROGRAM="/usr/bin/systemd-escape aaa-bbb-ccc", ENV{SYSTEMD_WANTS}+="test2@$result.service"
```
Then, a device expectedly gains a property:
```
SYSTEMD_WANTS=test1@foo\x2dbar\x2dbaz.service test2@aaa\x2dbbb\x2dccc.service
```
After the event being processed by udevd, PID1 processes the device, the
property previously was parsed with
`extract_first_word(EXTRACT_UNQUOTE)`, then the device unit gained the
following dependencies:
```
Wants=test1@foox2dbarx2dbaz.service test2@aaax2dbbbx2dccc.service
```
So both `%i` and `%I` for the template services did not match with the
original data, and it was hard to use `systemd-escape` in `PROGRAM=`
udev rule token.

This makes the property parsed with
`extract_first_word(EXTRACT_UNQUOTE|EXTRACT_RETAIN_ESCAPE)`, hence the
device unit now gains the following dependencies:
```
Wants=test1@foo\x2dbar\x2dbaz.service test2@aaa\x2dbbb\x2dccc.service
```
and `%I` for the template services match with the original data.

Fixes a bug caused by ceed8f0c8b9a46300eccd1afa2dd8d3c2cb6b47c (v233).

Fixes #16735.
Replaces #16737 and #35768.