Move all defaulting-of-CAP_FOO to sysconfig.h.

- The phrase `#ifdef CAP_` should never appear anywhere.

- The phrase `#ifndef CAP_` should appear only in sysconfig.h.

- The phrases `#if CAP_` and `#if !CAP_` may appear wherever,
    as long as "sysconfig.h" is included first.

The rules for `CAP_FOO` equally apply to `ISFOO`.

There are many one-off macros still tested with `#ifdef`,
including `HAVE_ACHIEVEMENTS`, `PRINT_ACHIEVEMENTS`,
`FAKEWEB`, `FAKE_SDL`, `EASY`, and `WHATEVER`. I don't
have much grasp on what these are used for or how they're
configured, so I'm leaving them alone.

help.cpp

@@ -357,7 +357,7 @@ EX string generateHelpForItem(eItem it) {
        " You need to go deep to collect lots of them.");
      }
      
-#if ISMOBILE==1
+#if ISMOBILE
    if(it == itOrbSafety)
      help += XLAT("This might be very useful for devices with limited memory.");
 #else
@@ -525,7 +525,7 @@ void addMinefieldExplanation(string& s) {
     );
 
   s += "\n\n";
-#if ISMOBILE==0
+#if !ISMOBILE
   s += XLAT("Known mines may be marked by pressing 'm'. Your allies won't step on marked mines.");
 #else
   s += XLAT("Known mines may be marked by touching while in drag mode. Your allies won't step on marked mines.");