Move AM_clearMarks call into P_InitNewLevel with other "subsystem wake
up" code.

Subversion-branch: /trunk
Subversion-revision: 2873
* Added P_CheckLevelName and P_CheckLevelMapNum variants of P_CheckLevel
  function
* Reformatted isMAPxy and isExMy macros
* Use ectype::isDigit in translation parser
* Potential bug fix in translation parser: do not I_Error in
  qstring::operator [] if length is 0
* Bit of C++ cleanup in w_levels

Subversion-branch: /trunk
Subversion-revision: 2874