shared/install: Preserve escape characters for escaped unit names

shared/install: Preserve escape characters for escaped unit names

Since switching to extract_first_word with no flags for parsing
unit names in 4c9565eea534cd233a913c8c21f7920dba229743, escape
characters will be stripped from escaped unit names such as
"mnt-persistent\x2dvolume.mount" resulting in the unit not being
configured as defined.  Preserve escape characters again for
compatibility with existing preset definitions.

(cherry picked from commit 82bd4da71e9cdd5a2e9266332f5a7399845e31f6)

Resolves: #1952686