xdg-autostart: avoid quadratic behaviour in strv parsing

xdg-autostart: avoid quadratic behaviour in strv parsing

The fuzzer test case has a giant line with ";;;;;;;;;;;..." which is turned into
a strv of empty strings. Unfortunately, when pushing each string, strv_push() needs
to walk the whole array, which leads to quadratic behaviour. So let's use
greedy_allocation here and also keep location in the string to avoid iterating.

build/fuzz-xdg-desktop test/fuzz/fuzz-xdg-desktop/oss-fuzz-22812  51.10s user 0.01s system 99% cpu 51.295 total
↓
build/fuzz-xdg-desktop test/fuzz/fuzz-xdg-desktop/oss-fuzz-22812  0.07s user 0.01s system 96% cpu 0.083 total

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=22812.

Other minor changes:
- say "was already defined" instead of "defined multiple times" to make it
  clear that we're ignoring this second definition, and not all definitions
  of the key
- unescaping needs to be done also for the last entry