tree-wide: replace strstr(a, b) == a with spa_strstartswith()

spa_strstartswith() is more immediately understandable.

Coccinelle spatch file:
	@@
	expression E1, E2;
	@@
	- strstr(E1, E2) != E1
	+ !spa_strstartswith(E1, E2)

	@@
	expression E1, E2;
	@@
	- strstr(E1, E2) == E1
	+ spa_strstartswith(E1, E2)

Applied to the tree except for alsa/acp/compat.h because it looks like
that header is still mostly as-is from PA.