Introduce strncmp_iw

Introduce strncmp_iw

The explicit locations completer patch will need a strncmp_iw
function, that to strcmp_iw like strncmp is to strcmp.  This patch
implements it.

(Unit tests added a bit further down in this series will exercise
this.)

gdb/ChangeLog:
2017-07-17  Pedro Alves  <palves@redhat.com>

	* utils.c (enum class strncmp_iw_mode): New.
	(strcmp_iw): Rename to ...
	(strncmp_iw_with_mode): ... this.  Add string2_len and mode
	parameters.  Handle them.
	(strncmp_iw): New.
	(strcmp_iw): Reimplement as wrapper around strncmp_iw_with_mode.
	* utils.h (strncmp_iw): Declare.
	(strcmp_iw): Move describing comments here.