Fix indentation of "maint print user-registers"

This fixes a failure of the test case "complete 'info registers '" in
completion.exp on architectures where the user registers have numbers
above 99.  In that case the output of "maint print user-registers" was
no longer indented, and the regexp in the test case failed to add them
to the list of expected completion results.  The fix also swaps the
columns "Name" and "Nr", such that the indentation is always the same,
and to be consistent with the output of "maint print registers".

gdb/ChangeLog:

	* user-regs.c (maintenance_print_user_registers): Swap "Nr" and
	"Name" columns.  Assure that the output is always indented.

gdb/testsuite/ChangeLog:

	* gdb.base/completion.exp: Adjust to format changes of "maint
	print user-registers".