mitogen: Send first stage as argv (786 bytes -> 796 bytes)

This saves one layer of quoting/quote escaping in the bootstrap command and a
string interpolation per connection. The cost is an increasing the bootstrap
command by 10 bytes. I like the tradeoff. I could be convinced to revert it.

```console
➜  mitogen git:(boot-cmd--argv) ✗ ./preamble_size.py
SSH command size: 796
Preamble (mitogen.core + econtext) size: 18230 (17.80KiB)

                        Original           Minimized           Compressed
mitogen.core         152237 148.7KiB  68453 66.8KiB 45.0%  18130 17.7KiB 11.9%
mitogen.parent        99181  96.9KiB  51384 50.2KiB 51.8%  12956 12.7KiB 13.1%
mitogen.fork           8445   8.2KiB   4139  4.0KiB 49.0%   1652  1.6KiB 19.6%
mitogen.ssh           10847  10.6KiB   6913  6.8KiB 63.7%   2102  2.1KiB 19.4%
mitogen.sudo          12089  11.8KiB   5924  5.8KiB 49.0%   2249  2.2KiB 18.6%
mitogen.select        12325  12.0KiB   2929  2.9KiB 23.8%    964  0.9KiB  7.8%
mitogen.service       41581  40.6KiB  22398 21.9KiB 53.9%   5847  5.7KiB 14.1%
mitogen.fakessh       15753  15.4KiB   8135  7.9KiB 51.6%   2672  2.6KiB 17.0%
mitogen.master        52891  51.7KiB  27586 26.9KiB 52.2%   7129  7.0KiB 13.5%
```

mitogen/parent.py

@@ -1488,7 +1488,8 @@ class Connection(object):
             '-c',
             'import sys;sys.path=[p for p in sys.path if p];'
             'import binascii,os,select,zlib;'
-            'exec(zlib.decompress(binascii.a2b_base64("%s",-15)))' % (encoded.decode(),),
+            'exec(zlib.decompress(binascii.a2b_base64(sys.argv[1]),-15))',
+            encoded.decode(),
         ]
 
     def get_econtext_config(self):