Modify cheri_ptrperm() to not clear the CHERI_PERM_GLOBAL bit when setting
other permission properties on a capability derived from a pointer.  This
is generally not what one wants -- and does suggest that the 'global' bit
is not quite a permission (per se), as with 'sealed'.
Annotate cheritest's invoke() function with __attribute__((cheri_ccall)),
and add unused sealed code and data capability pointers to represent $c1
and $c2.  This causes invoke() to properly clear $c3 on return.