New common function "startswith"

New common function "startswith"

This commit introduces a new inline common function "startswith"
which takes two string arguments and returns nonzero if the first
string starts with the second.  It also updates the 295 places
where this logic was written out longhand to use the new function.

gdb/ChangeLog:

	* common/common-utils.h (startswith): New inline function.
	All places where this logic was used updated to use the above.