[benfred#112] FreeBSD support

FreeBSD is a somewhat popular system among developers and it would be
nice to have it supported.
In order to support it, we have to patch the remoteprocess subcrate a
bit, as well as bump `proc-maps` and `read-process-memory`
crates version.

This change

[remoteprocess crate]

Adds FreeBSD support for

* process and thread locking (via ptrace)
* getting thread registers (via ptrace)
* getting process' executable and cwd (via libprocstat)
* read_process_memory contract has changed in the latest version, so
  OSX / Linux / Windows code has been adjusted accordingly.

The only thing that is left to be implemented is unwinder /
symbolificator. Therefore `py-spy` on FreeBSD does not support native
stack traces, yet.

[py-spy]

* Adds FreeBSD-specific versions of platform-dependent functions.
* Adds Cirrus CI configuration to run tests against FreeBSD
  platform. In order to set CI up, please refer to
  https://cirrus-ci.org/guide/quick-start/

.cirrus.yml

@@ -0,0 +1,18 @@
+freebsd_instance:
+  image: freebsd-12-0-release-amd64
+
+freebsd_task:
+  name: $TOOLCHAIN x86_64-unknown-freebsd
+  env:
+    matrix:
+      - TOOLCHAIN: stable
+      - TOOLCHAIN: beta
+      - TOOLCHAIN: nightly
+  setup_script:
+    - pkg install -y curl
+    - curl https://sh.rustup.rs -sSf --output rustup.sh
+    - sh rustup.sh -y --default-toolchain $TOOLCHAIN
+  build_script:
+    - $HOME/.cargo/bin/rustup run $TOOLCHAIN cargo build --verbose
+  test_script:
+    - $HOME/.cargo/bin/rustup run $TOOLCHAIN cargo test --verbose

Cargo.toml

@@ -24,7 +24,7 @@ libc = "0.2.34"
 log = "0.4"
 regex = "1"
 tempfile = "3.0.3"
-proc-maps = "0.1"
+proc-maps = { git = "https://github.com/akhramov/proc-maps", branch = "feature/freebsd-support" }
 memmap = "0.7.0"
 cpp_demangle = "0.2.12"
 rand = "0.6"