sysconf: Speed up _SC_PHYS_PAGES.

It's a silly waste of time to use the string-lookup version of
sysconf() rather than using the numeric version, even though that's
what Apple's code does.

Also adds comment explaining 32-bit issues.

Also, since the "i386" bug in Apple's code is really a 32-bit bug, the
feature-flag condition is adjusted to be more honest.  This has no
actual effect, since i386 is the only supported 32-bit processor in
the relevant OS versions.  This removes the only reference to
__MPLS_APPLE_I386__, but the definition is retained in case it's
useful in the future.

TESTED:
Passes tests on all platforms.

include/MacportsLegacySupport.h

@@ -262,7 +262,7 @@
 
 #define __MPLS_SDK_SUPPORT_SYSCONF_PHYS_PAGES__   (__MPLS_SDK_MAJOR < 101100)
 #define __MPLS_LIB_SUPPORT_SYSCONF_PHYS_PAGES__   (__MPLS_TARGET_OSVER < 101100 \
-                                                   || __MPLS_APPLE_I386__)
+                                                   || !__MPLS_64BIT)
 
 #define __MPLS_LIB_SUPPORT_SYSCONF_WRAP__ (__MPLS_LIB_SUPPORT_SYSCONF_NPROCESSORS__ \
                                            || __MPLS_LIB_SUPPORT_SYSCONF_PHYS_PAGES__)

src/sysconf.c

@@ -26,6 +26,7 @@
 #include <unistd.h>
 #include <dlfcn.h>
 #include <stdlib.h>
+#include <stddef.h>
 
 /*
  * Emulate several commonly used but missing (or broken) selectors from
@@ -75,10 +76,21 @@ long sysconf(int name) {
         /* the number of pages is the total memory / pagesize */
         uint64_t mem_size;
         size_t len = sizeof(mem_size);
+        int ms_mib[] = {CTL_HW, HW_MEMSIZE};
+        size_t ms_miblen = sizeof(ms_mib) / sizeof(ms_mib[0]);
         int pagesize = getpagesize();
 
-        sysctlbyname("hw.memsize", &mem_size, &len, NULL, 0);
+        if (sysctl(ms_mib, ms_miblen, &mem_size, &len, NULL, 0)) {
+          return -1;
+        }
 
+        /*
+         * Note that the 32-bit version of this function is inherently
+         * invalid in any system with >=1TB of RAM.
+         * Apple's 32-bit implementation is invalid in any system with
+         * >=2GB of RAM, which is why we override any existing Apple
+         * implementation in all 32-bit builds.
+         */
         return (long)(mem_size/pagesize);
 
     }