Flagsify EscapeStyle and make ESCAPE_BACKSLASH_ONELINE implicit

Flagsify EscapeStyle and make ESCAPE_BACKSLASH_ONELINE implicit

I want to tweak behaviour further, and that'll be easier when "style"
is converted to a bitfield.

Some callers used ESCAPE_BACKSLASH_ONELINE, and others not. But the
ones that didn't, simply didn't care, because the argument was assumed to
be one-line anyway (e.g. a service name). In environment-generator, this
could make a difference. But I think it's better to escape the newlines
there too. So newlines are now always escaped, to simplify the code and
the test matrix.