malloc(9): Document/add aligned variants

[adam_fenn.io]

Summary

As per https://reviews.freebsd.org/D29733#696995 and https://reviews.freebsd.org/D29733#696995, introduce malloc_aligned() and add this and malloc_domainset_aligned() to malloc(9).

Test Plan

Tested via a version of https://reviews.freebsd.org/differential/diff/91630/ with the following modification in kvm_clock_attach():

-	sc->timeinfos = malloc_domainset_aligned(round_page(mp_ncpus *
+	sc->timeinfos = malloc_aligned(round_page(mp_ncpus *
 	    sizeof(struct pvclock_vcpu_time_info)), PAGE_SIZE, M_DEVBUF,
-	    DOMAINSET_RR(), M_WAITOK | M_ZERO);
+	    M_WAITOK | M_ZERO);

[kib]

share/man/man9/malloc.9

79

You can use .Fo to avoid this long line

110

It would be useful to note that:

• align should be non-zero and power of two
• we only support align requests less or equal to the page size

[adam_fenn.io]

Address @kib's feedback.

[allanjude]

Man page changes approved