main: split out reading of /proc/sys/fs/nr_open into its own function

This doesn't really reduce the code size over all, but it does make main.c
shorter and more readable, and that's always a good thing.

src/basic/fd-util.h

@@ -108,3 +108,5 @@ static inline int make_null_stdio(void) {
         })
 
 int fd_reopen(int fd, int flags);
+
+int read_nr_open(void);