Allow tabs in environment files

bash allows them, and so should we.

string_has_cc is changed to allow tabs, and if they are not wanted,
they must be now checked for explicitly. There are two other callers,
apart from the env file loaders, and one already checked anyway, and
the other is changed to check.

https://bugs.freedesktop.org/show_bug.cgi?id=68592
https://bugs.gentoo.org/show_bug.cgi?id=481554
man: fix description of file order application