ls: don't form or compute the length of strings that won't be used.

ls: don't form or compute the length of strings that won't be used.
* src/ls.c (gobble_file): Form and compute length of strings for
block size, owner, group, author, major+minor numbers and file size
only if they'll actually be used.  I.e., don't form most of them
when not producing long format output.