basic: split out update_reboot_parameter_and_warn() into its own .c/.h files

This is primarily preparation for a follow-up commit that adds a common
implementation of the other side of the reboot parameter file, i.e. the
code that reads the file and issues reboot() for it.

src/basic/meson.build

@@ -158,6 +158,8 @@ basic_sources = files('''
         ratelimit.h
         raw-clone.h
         raw-reboot.h
+        reboot-util.c
+        reboot-util.h
         refcnt.h
         replace-var.c
         replace-var.h