SSH2: simplify _send_channel_packet

the last _send_channel_packet was unnecessarily complex. this lead
to a number of problems. for example,

the window size was checked for adjustments in two places. in the
second place it receives a window adjustment packet and just assumes
that the window adjustment packet was big enough for the rest of the
data.

(thanks, pixall!)
Merge pull request #538 from terrafrost/window-size-changes

SSH2: simplify _send_channel_packet

* terrafrost/window-size-changes:
  SSH2: simplify _send_channel_packet
Merge branch 'master' into php5

* master:
  SSH2: simplify _send_channel_packet
Add public keyword to Crypt\Random::string().
Merge pull request #542 from bantu/fix-540

Add public keyword to Crypt\Random::string().

* bantu/fix-540:
  Add public keyword to Crypt\Random::string().
Refactor defined MATH_BIGINTEGER_ constants into class constants.
Move in-code comments to property doc blocks.
Add #@+ block for properties used by internal implementation.
Merge pull request #544 from bantu/cnelissen-BigIntegerConstantsFix

Refactor defined MATH_BIGINTEGER_ constants into class constants.

* bantu/cnelissen-BigIntegerConstantsFix:
  Add #@+ block for properties used by internal implementation.
  Move in-code comments to property doc blocks.
  Refactor defined MATH_BIGINTEGER_ constants into class constants.
Merge branch 'php5' of https://github.com/phpseclib/phpseclib into php5