dbghelp: Live target detection.

- DBGHELP_NOLIVE env variable is tested earlier to avoid 
  some strange effects in live target detection
- in WineDbg, delete DBGHELP_NOLIVE before attaching to
  a process, because we know it must be a live target

dlls/dbghelp/dbghelp.c

@@ -170,8 +170,8 @@ static BOOL WINAPI process_invade_cb(char* name, DWORD base, DWORD size, void* u
 static BOOL check_live_target(struct process* pcs)
 {
     if (!GetProcessId(pcs->handle)) return FALSE;
-    if (!elf_read_wine_loader_dbg_info(pcs)) return FALSE;
-    return getenv("DBGHELP_NOLIVE") == NULL;
+    if (GetEnvironmentVariableA("DBGHELP_NOLIVE", NULL, 0)) return FALSE;
+    return elf_read_wine_loader_dbg_info(pcs);
 }
 
 /******************************************************************

programs/winedbg/tgt_active.c

@@ -83,6 +83,8 @@ BOOL dbg_attach_debuggee(DWORD pid, BOOL cofe, BOOL wfe)
     }
     dbg_curr_process->continue_on_first_exception = cofe;
 
+    SetEnvironmentVariableA("DBGHELP_NOLIVE", NULL);
+
     if (wfe) /* shall we proceed all debug events until we get an exception ? */
     {
         dbg_interactiveP = FALSE;