Add completer for skip numbers

Add completer for skip numbers

Add completer to various commands that accept skip numbers:

  - skip enable
  - skip disable
  - skip delete
  - info skip

These commands also accept ranges, the completer works for that but is
not very smart.  It will suggest invalid ranges, for example when doing
"2-<TAB>" it will suggest "1", which would not result in a valid range.
Also, it will keep suggesting when doing "1-2-<TAB>", even though it's
an invalid syntax.

A future idea would be to make a re-usable and well-tested completer for
numbers and ranges.  I think it could at least be re-used for breakpoint
and thread numbers (for example with the "enable breakpoints" command).

gdb/ChangeLog:

	* skip.c (complete_skip_number): New function.
	(_initialize_step_skip): Add completers to some skip commands.

gdb/testsuite/ChangeLog:

	* gdb.base/skip.exp: Add standard_testfile.  Add "skip delete"
	completer tests.