wc: with only --bytes, determine size more efficiently

wc: with only --bytes, determine size more efficiently

* src/wc.c (wc): Avoid reading the end of the file
when the size is not a multiple of PAGE_SIZE,
as the special case handling for files in /proc and /sys
is only required when st_size is 0 or a multiple of PAGE_SIZE.
* tests/misc/wc-proc.sh: Add a test case.