0.9.2.26: refactoring internals of foreign linkage

  * rename FOREIGN-SYMBOL-ADDRESS => FOREIGN-SYMBOL-SAP,
     FOREIGN-SYMBOL-ADDRESS-AS-INTEGER => FOREIGN-SYMBOL-ADDRESS, and
     so forth. Follow this scheme consistently, calling foreign saps
     saps, and addresses addresses.
  * split GET-DYNAMIC-FOREIGN-SYMBOL-ADDRESS into
     FIND-DYNAMIC-FOREIGN-SYMBOL-ADDRESS and
     ENSURE-DYNAMIC-FOREIGN-SYMBOL-ADDRESS.
  * new function: FIND-FOREIGN-SYMBOL-ADDRESS, which doesn't enter
     the symbol to linkage table. Use it in SB-POSIX to detect the
     presence of foreign symbols.
  * merge patch by David Lichteblau:
     ENSURE-DYNAMIC-FOREIGN-SYMBOL-ADDRESS keeps track of used foreign
     symbols in hash-tables instead of lists.