lang/rust and lang/rust-nightly: Build-conflict with themselves

A newer version of Rust fails to build if an older version is installed
because the build process picks libraries in %%LOCALBASE%%/lib before
those from the build directory.

In the pkg-plist of both ports, x86_64-unknown-freebsd is now a
variable automatically set in the Makefile. This avoids the need for a
separate port for DragonFlyBSD. [1]

Still in the pkg-plist, RUST_VSN_HASH is automatically computed in the
lang/rust's Makefile, like it was already done for lang/rust-nightly.

lang/rust-nightly USES libedit. patch-mk_main.mk was copied from
lang/rust so the correct library is picked (ie. the one from Ports, not
the one from the base). This was already fixed in lang/rust.

lang/rust includes bsd.port.options.mk and bsd.port.mk, instead of
bsd.port.pre.mk and bsd.port.post.mk. This was already fixed in
lang/rust-nightly.

Both ports are now closer to each other.

PR: 202869 [1]
Submitted by: Michael Neumann <mneumann@ntecs.de> [1]
Reviewed by: kwm
Approved by: kwm
Differential Revision: https://reviews.freebsd.org/D3234