Add and use a new NumEntryGuards consensus parameter.

When specified, it overrides our default of 3 entry guards.

(By default, it overrides the number of directory guards too.)

Implements ticket 12688.
add a NumDirectoryGuards consensus param too
update manpage for numentryguards / numdirectoryguards
Merge branch 'maint-0.2.4' into maint-0.2.5