Previously the global context was either accessed via gopts.ctx,
stored in a local variable and then used within that function or
sometimes both. This makes it very hard to follow which ctx or a wrapped
version of it reaches which method.
Thus just drop the context from the globalOptions struct and pass it
explicitly to every command line handler method.
The standard UNIX-style ordering of command-line arguments places
optional flags before other positional arguments. All of restic's
commands support this ordering, but some of the usage strings showed the
flags after the positional arguments (which restic also parses just
fine). This change updates the doc strings to reflect the standard
ordering.
Because the `restic help` command comes directly from Cobra, there does
not appear to be a way to update the argument ordering in its usage
string, so it maintains the non-standard ordering (positional arguments
before optional flags).