Terminal: Center Find window in term window frame

... instead of positioning it under the mouse pointer as suggested
by Axel.
libroot: make all areas executable for old binaries

 * If at least one image is either B_HAIKU_ABI_GCC_2_ANCIENT or
   B_HAIKU_ABI_GCC_2_BEOS almost all areas are marked as executable.
 * B_EXECUTE_AREA and B_STACK_AREA are made public. The former is enforced since
   the introduction of DEP and apps need it to correctly set area protection.
   The latter is currently needed only to recognize stack areas and fix their
   protection in compatibility mode, but may also be useful if an app wants
   to use sigaltstack from POSIX API.