switchroot: Replace custom error printing with err/warn functions from libc

switchroot: Replace custom error printing with err/warn functions from libc

The `warn()` libc extension has exactly the same behaviour as our own
`perrorv` function, but is available in (at least) glibc and musl.  As an
added bonus the similar function `err()` which will exit with an error
code afterwards.

This implementation is tidier and allows us to get rid of our own
`perrorv`.  It paves the way to removing `ostree-mount-util.c` to simplify
the build scripts.

Original idea by @cgwalters in #477.

Closes: #478
Approved by: cgwalters