vfs: vn_dir_next_dirent(): Simplify interface and harden

Simplify the old interface (one less argument, simpler termination test)
and add documentation about it. Add more sanity checks (mostly under
INVARIANTS, but also in the general case to prevent infinite
loops). Drop the explicit test on minimum directory entry size (without
INVARIANTS).

Deal with the impacts in callers (dirent_exists() and vop_stdvptocnp()).
dirent_exists() has been simplified a bit, preserving the exact same
semantics but for the return code whose meaning has been reversed (0 now
means the entry exists, ENOENT that it doesn't and other values are
genuine errors). While here, suppress gratuitous casts of malloc return
values.

vn_dir_next_dirent() has been tested by a 'make -j4 buildkernel' with a
temporary modification to the VFS cache causing vn_vptocnp() to always
call VOP_VPTOCNP() and finally vop_stdvptocnp() (observed with temporary
debug counters).

Export new _GENERIC_MINDIRSIZ and _GENERIC_MAXDIRSIZ on __BSD_VISIBLE,
and GENERIC_MINDIRSIZ and GENERIC_MAXDIRSIZ on _KERNEL.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39764