make recording pending signals re-entrant

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on August 11, 2009 01:18:30_

We no longer block SUSPEND_SIGNAL (issue #184) or SIGSEGV (issue #193).
But we can have re-entrancy issues in this routine if the app uses the same
SUSPEND_SIGNAL, or the nested SIGSEGV needs to be sent to the app. The
latter shouldn't happen unless the app sends SIGSEGV via SYS_kill().
To avoid re-entrant execution of special_heap_alloc() and of
prepending to the pending list we just drop this signal.
This Issue covers supporting re-entrant recording.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=194_