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!

src/core/executor.c

@@ -245,6 +245,6 @@ int run_executor(int argc, char *argv[]) {
         return r < 0 ? EXIT_FAILURE : r;
 }
 
-#if !BUILD_EXECUTOR_SINGLE
+#if !SYSTEMD_MULTICALL_BINARY
 _alias_(run_executor) main;
 #endif

src/core/main.c

@@ -3932,7 +3932,7 @@ finish:
 }
 
 int main(int argc, char *argv[]) {
-#if BUILD_EXECUTOR_SINGLE
+#if SYSTEMD_MULTICALL_BINARY
         if (invoked_as(argv, "executor"))
                 return run_executor(argc, argv);
 #endif

src/core/manager.c

@@ -900,7 +900,7 @@ static int pin_executor_binary(int *ret_fd) {
 
         assert(ret_fd);
 
-#if BUILD_EXECUTOR_SINGLE
+#if SYSTEMD_MULTICALL_BINARY
         int r;
 
         r = open_and_check_executable("/proc/self/exe", /* root= */ NULL, &path, ret_fd);

src/core/meson.build

@@ -209,11 +209,9 @@ systemd_deps = [
         libselinux_cflags,
 ]
 
-link_executor_shared = get_option('link-executor-shared')
+executor_libs = get_option('link-executor-shared') ? core_libs_shared : core_libs_static
 
-executor_libs = link_executor_shared == 'true' ? core_libs_shared : core_libs_static
-
-if link_executor_shared == 'single'
+if conf.get('SYSTEMD_MULTICALL_BINARY') == 1
         systemd_sources += systemd_executor_sources
         systemd_deps += [
                 libbpf_cflags,
@@ -256,7 +254,7 @@ executables += [
         },
 ]
 
-if link_executor_shared == 'single'
+if conf.get('SYSTEMD_MULTICALL_BINARY') == 1
         # Symlink for external callers
         install_symlink('systemd-executor',
                         pointing_to : 'systemd',