Use uintptr_t to convert a pointer to an integer

Issue seen on CheriBSD running on a Morello board
(capability-to-integer-cast error).

The relevant capability metadata for pointers is preserved in the
uintptr_t datatype.  This way, afterwards in the code, the other way
round of converting the computed integer value to a pointer works fine.

configure.ac

@@ -653,6 +653,7 @@ AC_TYPE_OFF_T
 AC_TYPE_PID_T
 AC_TYPE_SIZE_T
 AC_TYPE_SSIZE_T
+AC_TYPE_UINTPTR_T
 AC_FUNC_GETGROUPS
 AC_TYPE_UID_T
 AC_CHECK_TYPES([sig_atomic_t], [], [],

lib/mmap.c

@@ -26,8 +26,8 @@ inn__msync_page(void *p, size_t length, int flags)
         return -1;
     } else {
         const size_t mask = ~(size_t) (pagesize - 1);
-        char *start = (char *) ((size_t) p & mask);
-        char *end = (char *) (((size_t) p + length + pagesize) & mask);
+        char *start = (char *) ((uintptr_t) p & mask);
+        char *end = (char *) (((uintptr_t) p + length + pagesize) & mask);
 
         return msync(start, end - start, flags);
     }

storage/cnfs/cnfs.c

@@ -1004,8 +1004,8 @@ cnfs_mapcntl(void *p, size_t length, int flags)
 {
     char *start, *end;
 
-    start = (char *) ((size_t) p & ~(size_t) (pagesize - 1));
-    end = (char *) ((size_t) ((char *) p + length + pagesize)
+    start = (char *) ((uintptr_t) p & ~(size_t) (pagesize - 1));
+    end = (char *) (((uintptr_t) p + length + pagesize)
                     & ~(size_t) (pagesize - 1));
     if (flags == MS_INVALIDATE) {
         msync(start, end - start, flags);