libutil: Support getSelfExe on FreeBSD

getSelfExe is used in a few places re-execute nix.
Current code in this file uses ifdefs to support several
platforms, just keep doing that

Change-Id: Iecc2ada0101aea0c30524e3a1218594f919d74bf

src/libutil/current-process.cc

@@ -15,6 +15,11 @@
 # include <sys/resource.h>
 #endif
 
+#if __FreeBSD__
+# include <sys/param.h>
+# include <sys/sysctl.h>
+#endif
+
 #include <sys/mount.h>
 #include <cgroup.hh>
 
@@ -102,6 +107,24 @@ std::optional<Path> getSelfExe()
             return buf;
         else
             return std::nullopt;
+        #elif __FreeBSD__
+        int sysctlName[] = {
+            CTL_KERN,
+            KERN_PROC,
+            KERN_PROC_PATHNAME,
+            -1,
+        };
+        size_t pathLen = 0;
+        if (sysctl(sysctlName, sizeof(sysctlName) / sizeof(sysctlName[0]), nullptr, &pathLen, nullptr, 0) < 0) {
+	        return std::nullopt;
+        }
+
+        std::vector<char> path(pathLen);
+        if (sysctl(sysctlName, sizeof(sysctlName) / sizeof(sysctlName[0]), path.data(), &pathLen, nullptr, 0) < 0) {
+            return std::nullopt;
+        }
+
+        return Path(path.begin(), path.end());
         #else
         return std::nullopt;
         #endif