boot - Fix minor memory leak, statistics
* open() calls devopen() which installs a generic malloc'd device
wrapper in f->f_devdata. However, some devices will replace this
with their own descriptor.
Add F_DEVDESC to indicate when f->f_devdata contains this generic
object, free and clear the generic object when replacing f_devdata
in particular devices.
This fixes a minor memory leak of 192 bytes on open()s made by the boot
code when booting from e.g. PXE.
* Fix incorrect MallocCount statistics. calloc() and realloc() were
improperly incrementing the global, even though the malloc() call they
both make also increments it.
* Remove the unused F_NODEV flag. This flag is never set, collapse the
related code.