Fix a very long-standing error in handling .SHELL targets: this target

Fix a very long-standing error in handling .SHELL targets: this target
uses the brk_string function to parse the line. That function uses static
storage for both the expanded string and the returned argv[] vector.
The JobParseShell function simply stored away pointers into this static
storage. On the next use of something like ${FOO:O} this storage would
get overwritten with fatal results.

This also allows us to make the shells[] array const bringing us one step
further in making make WARNS=4 ready.