depmod: Add better error messages when facing loops

depmod: Add better error messages when facing loops

Since now depmod fails when there are module loops, let's at least give
better error messages, printing the loops we found. Since we may have
more than 1 loop, just printing the modules that are in loop is not
very clear.

Assuming as an example 2 independent loops, this is how the new messages
compare to the old ones:

Before:
	depmod: ERROR: Found 5 modules in dependency cycles!
	depmod: ERROR: /tmp/test-kmod//lib/modules/3.14.4-1-ARCH/kernel/moduleE.ko in dependency cycle!
	depmod: ERROR: /tmp/test-kmod//lib/modules/3.14.4-1-ARCH/kernel/moduleB.ko in dependency cycle!
	depmod: ERROR: /tmp/test-kmod//lib/modules/3.14.4-1-ARCH/kernel/moduleC.ko in dependency cycle!
	depmod: ERROR: /tmp/test-kmod//lib/modules/3.14.4-1-ARCH/kernel/moduleD.ko in dependency cycle!
	depmod: ERROR: /tmp/test-kmod//lib/modules/3.14.4-1-ARCH/kernel/moduleA.ko in dependency cycle!

After:
	depmod: ERROR: Found 5 modules in dependency cycles!
	depmod: ERROR: Cycle detected: moduleE -> moduleD -> moduleE
	depmod: ERROR: Cycle detected: moduleB -> moduleC -> moduleA -> moduleB