Merge #1561

1561: Declare the MSRV in Cargo.toml r=rtzoeller a=asomers

This is a new feature in Cargo 1.56.0, currently in beta.  Once Nix's
MSRV is >= 1.56.0, this feature will prevent future problems like the
bitflags 1.3.0 fiasco.

Issue #1491
Issue #1510
Issue #1548
Issue #1555

Co-authored-by: Alan Somers <asomers@gmail.com>

Cargo.toml

@@ -3,6 +3,7 @@ name        = "nix"
 description = "Rust friendly bindings to *nix APIs"
 edition     = "2018"
 version     = "0.23.0"
+rust-version = "1.46"
 authors     = ["The nix-rust Project Developers"]
 repository  = "https://github.com/nix-rust/nix"
 license     = "MIT"