Fail if boolean env vars are set with common negative strings

Fail if boolean env vars are set with common negative strings

Boolean environment variables have strange semantics: if the variable
is set then the configuration setting is considered "true".  This means
users can easily be mistaken that eg CCACHE_DISABLE=false means that
the "disable" setting is set to false.

To avoid too much backwards-incompatibility, we now exit with an error if
some common negative-sounding (case-insensitive) values:
"0", "false", "disable", "no".

All other values (including the empty string) are still considered to mean
"true".

Resolves https://github.com/ccache/ccache/issues/182