Show a better error message for mount(2) -> ENOSPC

[smcv]

• utils: Give warnv() the ability to log strerror() or similar

  This lets die_with_error() reuse it.

• utils: Add a specialized function to print errno for mount(2)

  mount(2) uses ENOSPC to represent an arbitrary anti-denial-of-service limit being exceeded, which is outside the usual meaning of "No space left on device". We can make this clearer by catching that particular failure mode and giving users a hint.

  Reference: https://bugzilla.kernel.org/show_bug.cgi?id=218336
  Resolves: https://github.com/ValveSoftware/steam-runtime/issues/637

cc @RyuzakiKK

[smcv]

(Prior art: https://sourceforge.net/projects/libexplain/, but that doesn't seem to be actively maintained, and doesn't know that mount(2) can fail with ENOSPC.)