all-io: rename read_all/write_all to ul_read_all/ul_write_all

all-io: rename read_all/write_all to ul_read_all/ul_write_all

Add ul_ namespace prefix to all helper functions in all-io.h:

  write_all()      → ul_write_all()
  read_all()       → ul_read_all()
  read_all_alloc() → ul_read_all_alloc()
  fwrite_all()     → ul_fwrite_all()
  sendfile_all()   → ul_sendfile_all()

This is a mechanical rename with no logic changes, preparing
for the VFS I/O abstraction layer.

Addresses: https://github.com/util-linux/util-linux/issues/4308
Signed-off-by: Karel Zak <kzak@redhat.com>