shared: Add a linker script so that all functions are tagget @SD_SHARED instead of @Base (#6669)

This helps prevent symbol collisions with other programs and libraries. In particular,
because PAM modules are loaded into the process that is creating the session, and
systemd creates PAM sessions, the potential for collisions is high.

Disambiguate all systemd calls by tagging a 'version' SD_SHARED.

Fixes #6624

src/shared/libshared.sym

@@ -0,0 +1,3 @@
+SD_SHARED {
+	global: *;
+};

src/shared/meson.build

@@ -136,6 +136,8 @@ libshared_deps = [threads,
                   liblz4,
                   libblkid]
 
+libshared_sym_path = '@0@/libshared.sym'.format(meson.current_source_dir())
+
 libshared = shared_library(
         libshared_name,
         shared_sources,
@@ -144,7 +146,8 @@ libshared = shared_library(
         libsystemd_internal_sources,
         libudev_sources,
         include_directories : includes,
-        link_args : ['-shared'],
+        link_args : ['-shared',
+                    '-Wl,--version-script=' + libshared_sym_path],
         c_args : ['-fvisibility=default'],
         dependencies : libshared_deps,
         install : true,