Change the algorithm that matches the builtin shells from the name keyword

Change the algorithm that matches the builtin shells from the name keyword
of the .SHELL target. Formerly it used to select the shell with the
longest common trailing substring, so that bash would select sh, but pocsh
would select csh. Now an exact match is required so that specifying bash
without also giving a path and the other keywords will give an error.

PR:
Submitted by:
Reviewed by:	ru
Approved by:
Obtained from:
MFC after: