meson: replace -Dbuild-executor-shared=single with -Dsystemd-multicall-binary=true

This effectively reverts the meson config changes introduced in
0c6186695a. I wasn't happy with that form
back when it was commited, and in hindsight, I like it even less. Let's
just add a new option that hopefully is easier to understand and will
work better when new functionality is added in the future.

Now there are two options:
link-executor-shared (boolean, defaults to true)
  — "link (separate) systemd-executor to libsystemd-shared.so and libsystemd-core.so"
systemd-multicall-binary (also boolean, defaults to false)
  — "link systemd+systemd-executor as a single binary"

Build directories will need to be recreated again, sorry!

meson.build

@@ -78,8 +78,7 @@ conf.set10('FUZZ_USE_SIZE_LIMIT', fuzzer_build)
 # We'll set this to '1' for EFI builds in a different place.
 conf.set10('SD_BOOT', false)
 
-link_executor_shared = get_option('link-executor-shared')
-conf.set10('BUILD_EXECUTOR_SINGLE', link_executor_shared == 'single')
+conf.set10('SYSTEMD_MULTICALL_BINARY', get_option('systemd-multicall-binary'))
 
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