change string indexing to avoid invalid indexes in more cases, and
eliminate the thisind() function

For example, search("∀ x", "∀", 1) used to give 1:3, which doesn't
make much sense since the "3" is an artifact of the encoding and isn't
a valid index. Now it gives 1:1, or in general a:b where a is the index
of the first found character and b is the index of the last found character.
Merge branch 'master' of github.com:JuliaLang/julia into jb/strindexchanges
Merge branch 'master' of github.com:JuliaLang/julia into jb/strindexchanges

Conflicts:
	base/exports.jl
Merge branch 'master' of github.com:JuliaLang/julia into jb/strindexchanges

Conflicts:
	base/exports.jl
Merge branch 'jb/strindexchanges' of github.com:JuliaLang/julia into jb/strindexchanges
Merge branch 'master' of github.com:JuliaLang/julia into jb/strindexchanges
Merge pull request #3770 from JuliaLang/jb/strindexchanges

change string indexing to avoid invalid indexes