Process: Fixed multiple issues with search_path().

Unified handling of $PATH.

  * Added path_separator.

  * search_path() now invalidates the cached path if $PATH is changed.

  * search_path() now uses locate_binary() to scan the path.

  * Moved an __NT__ special case from locate_binary() to search_path().

  * spawn_pike() now uses search_path().
Compiler (amd64): Fixed bug in F_POS_INT_INDEX.

The range check in F_POS_INT_INDEX used the wrong comparison opcode
which caused indexing of arrays with their size to be allowed.

Added some corresponding tests to the testsuite.

Thanks to Stewa for the report.