[WIP] Removing code duplication in vim_strchr()

Logic defined in vim_strbyte() is duplicated at the end of vim_strchr().
Replacing duplicated code with a call to vim_strbyte().
Removing code duplication in vim_strchr()

Logic defined in vim_strbyte() is duplicated at the end of vim_strchr().
Replacing duplicated code with a call to vim_strbyte().
nonnull arg function attribute on vim_strchr()
nonnull arg function attribute on vim_strbyte()
Merge branch 'vim_strchr_cleanup'