discontinue use of alloca

NetBSD emits:
warning: Warning: reference to the libc supplied alloca(3); this most likely will not work. Please use the compiler provided version of alloca(3), by supplying the appropriate compiler flags (e.g. not -std=c89).

and man 3 alloca says:

       Normally,  gcc(1)  translates  calls  to  alloca()  with  inlined code.  This is not done when either the -ansi, -std=c89, -std=c99, or the
       -std=c11 option is given and the header <alloca.h> is not included.  Otherwise, (without an -ansi or -std=c* option) the glibc  version  of
       <stdlib.h> includes <alloca.h> and that contains the lines:

           #ifdef  __GNUC__
           #define alloca(size)   __builtin_alloca (size)
           #endif

It looks like alloca is a bad idea in modern C/C++, so we use
VLAs for C and std::vector for C++.

src/common/spawn.cpp

@@ -91,7 +91,7 @@ int spawn(const char *filename, const std::vector<std::string>& args, bool wait)
   MINFO("Child exited with " << exitCode);
   return static_cast<int>(exitCode);
 #else
-  char **argv = (char**)alloca(sizeof(char*) * (args.size() + 1));
+  std::vector<char*> argv(args.size() + 1);
   for (size_t n = 0; n < args.size(); ++n)
     argv[n] = (char*)args[n].c_str();
   argv[args.size()] = NULL;
@@ -109,7 +109,7 @@ int spawn(const char *filename, const std::vector<std::string>& args, bool wait)
     tools::closefrom(3);
     close(0);
     char *envp[] = {NULL};
-    execve(filename, argv, envp);
+    execve(filename, argv.data(), envp);
     MERROR("Failed to execve: " << strerror(errno));
     return -1;
   }