Use fewer #ifdefs for dynamic vcaches

Use fewer #ifdefs for dynamic vcaches

When we're not in AFS_MAXVCOUNT_ENV, make afsd_dynamic_vcaches a
static 0, which allows the removal of a scattering of #ifdef's in
the middle of conditionals in afs_vcache.c, and generally improves
the code browsing experience.

Also, move the externs for this variable to afs.h, where they belong,
and fix related formatting.

Change-Id: I4d56ebae63fe1855ac80f8511b9fc76f7e5a326f
Reviewed-on: http://gerrit.openafs.org/741
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>