* osdata.c (get_osdata): Warn separately if target does not report
type list.
(info_osdata_command): Allow empty type, report error if target
does not return available types of OS data.
* linux-nat.c (linux_nat_xfer_osdata): Report list of OS data
types if no annex supplied.
* gdb.texinfo (Operating System Auxiliary Information): Describe
"info os" when no arguments given.