Adds support for OpenBSD.

In order to download and unpack node source files under OpenBSD it is
necessary to run the extraction into two steps. Since `tar` and `xz` are
not integrated, the `-J` option is not available on OpenBSD default
`tar` tool.

This commit adds a test in order to figure this out, and handles the
proper file extraction under OpenBSD. The previous behaviour for other
Operating Systems should be untouched (more testing is welcome).

This commit does not yet handles the next issue, which is to compile
certain versions of node under OpenBSD. This issue is a bit more
delicated, since OpenBSD uses LibreSSL, and node support for it is still
not complete.

 - Adds check for tar's support for J option

nvm.sh

@@ -1606,6 +1606,7 @@ nvm_get_os() {
     Darwin\ *) NVM_OS=darwin ;;
     SunOS\ *) NVM_OS=sunos ;;
     FreeBSD\ *) NVM_OS=freebsd ;;
+    OpenBSD\ *) NVM_OS=openbsd ;;
     AIX\ *) NVM_OS=aix ;;
   esac
   nvm_echo "${NVM_OS-}"
@@ -1933,6 +1934,69 @@ nvm_download_artifact() {
   nvm_echo "${TARBALL}"
 }
 
+nvm_tar_knows_J() {
+  ERR_MSG=$(tar -J 2>&1 | head -1)
+  [ "_$ERR_MSG" != "_tar: unknown option -- J" ]
+}
+
+
+# args: tarball path, NVM_OS var
+# returns: a string with the path of the extracted compressed file
+nvm_extract_tar_artifact() {
+  local tar_compression_flag
+  local TARBALL
+  local TAR_COMMAND_SUFFIX
+  local TMPDIR
+  local NVM_OS
+  local tar
+
+  TARBALL="${1}"
+  NVM_OS="${2}"
+
+  tar='tar'
+  if [ "${NVM_OS}" = 'aix' ]; then
+    tar='gtar'
+  fi
+
+  tar_compression_flag='z'
+
+  if ! nvm_tar_knows_J && nvm_has xz && nvm_supports_xz "${VERSION}"; then
+
+    if [ -f "${TARBALL}" ]; then
+      command xz -dk "${TARBALL}"
+    fi
+
+    TARBALL=${TARBALL%.xz}
+    tar_compression_flag=
+    TAR_COMMAND_SUFFIX=
+    TMPDIR="$(dirname "${TARBALL}")"
+
+  else
+
+    if nvm_tar_knows_J; then
+      tar_compression_flag='J'
+    fi
+
+    TAR_COMMAND_SUFFIX="--strip-components 1"
+    TMPDIR="$(dirname "${TARBALL}")/files"
+  fi
+
+  if [ -f "${TARBALL}" ] && [ -d "${TMPDIR}" ]; then
+    # shellcheck disable=SC2086
+    command ${tar} "-x${tar_compression_flag}f" "${TARBALL}" -C "${TMPDIR}" ${TAR_COMMAND_SUFFIX}
+  fi
+
+  # The `-C` option for `tar` on Openbsd takes its parameter and extracts files
+  # from the tarball into a new directory named after it. That's why this update
+  # in the `$TMPDIR` is necessary
+  if [ "_$NVM_OS" = "_openbsd" ]; then
+    BASE=$(basename "${TMPDIR}")
+    TMPDIR="${TMPDIR}/${BASE}"
+  fi
+
+  nvm_echo "$TMPDIR"
+}
+
 nvm_get_make_jobs() {
   if nvm_is_natural_num "${1-}"; then
     NVM_MAKE_JOBS="$1"
@@ -2030,7 +2094,7 @@ nvm_install_source() {
       make='gmake'
       MAKE_CXX="CC=${CC:-cc} CXX=${CXX:-c++}"
     ;;
-    'darwin')
+    'openbsd' | 'darwin')
       MAKE_CXX="CC=${CC:-cc} CXX=${CXX:-c++}"
     ;;
     'aix')
@@ -2044,18 +2108,6 @@ nvm_install_source() {
     fi
   fi
 
-  local tar_compression_flag
-  tar_compression_flag='z'
-  if nvm_supports_xz "${VERSION}"; then
-    tar_compression_flag='J'
-  fi
-
-  local tar
-  tar='tar'
-  if [ "${NVM_OS}" = 'aix' ]; then
-    tar='gtar'
-  fi
-
   local TARBALL
   local TMPDIR
   local VERSION_PATH
@@ -2071,11 +2123,8 @@ nvm_install_source() {
 
   TARBALL="$(PROGRESS_BAR="${PROGRESS_BAR}" nvm_download_artifact "${FLAVOR}" source "${TYPE}" "${VERSION}" | command tail -1)" && \
   [ -f "${TARBALL}" ] && \
-  TMPDIR="$(dirname "${TARBALL}")/files" && \
+  TMPDIR="$(nvm_extract_tar_artifact "${TARBALL}" "${NVM_OS}" | command tail -1)" && \
   if ! (
-    # shellcheck disable=SC2086
-    command mkdir -p "${TMPDIR}" && \
-    command "${tar}" -x${tar_compression_flag}f "${TARBALL}" -C "${TMPDIR}" --strip-components 1 && \
     VERSION_PATH="$(nvm_version_path "${PREFIXED_VERSION}")" && \
     nvm_cd "${TMPDIR}" && \
     nvm_echo '$>'./configure --prefix="${VERSION_PATH}" $ADDITIONAL_PARAMETERS'<' && \