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/util.h

@@ -186,8 +186,6 @@ uint64_t physical_memory_scale(uint64_t v, uint64_t max);
 uint64_t system_tasks_max(void);
 uint64_t system_tasks_max_scale(uint64_t v, uint64_t max);
 
-int update_reboot_parameter_and_warn(const char *param);
-
 int version(void);
 
 int str_verscmp(const char *s1, const char *s2);