os/*_defs.h: MAXNAMLEN cleanup

For non-Windows systems, we assume that NAME_MAX is in <limits.h>, as
specified in POSIX.1-2008[1]. For Windows, which doesn't have NAME_MAX,
just define it ourselves to _MAX_PATH[2].

Also, remove two (now unused) HAVE_*_H checks.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html
[2]: https://msdn.microsoft.com/en-us/library/930f87yf.aspx

Helped-by: Seth Jackson

src/nvim/os/os_defs.h

@@ -13,28 +13,7 @@
 # include "nvim/os/unix_defs.h"
 #endif
 
-#if defined(DIRSIZ) && !defined(MAXNAMLEN)
-# define MAXNAMLEN DIRSIZ
-#endif
-
-#if defined(UFS_MAXNAMLEN) && !defined(MAXNAMLEN)
-# define MAXNAMLEN UFS_MAXNAMLEN    /* for dynix/ptx */
-#endif
-
-#if defined(NAME_MAX) && !defined(MAXNAMLEN)
-# define MAXNAMLEN NAME_MAX         /* for Linux before .99p3 */
-#endif
-
-#if defined(_MAX_PATH) && !defined(MAXNAMLEN)
-# define MAXNAMLEN _MAX_PATH       /* for Windows */
-#endif
-
-// Default value.
-#ifndef MAXNAMLEN
-# define MAXNAMLEN 512
-#endif
-
-#define BASENAMELEN (MAXNAMLEN - 5)
+#define BASENAMELEN (NAME_MAX - 5)
 
 // Use the system path length if it makes sense.
 #if defined(PATH_MAX) && (PATH_MAX > 1000)

src/nvim/os/unix_defs.h

@@ -1,13 +1,12 @@
 #ifndef NVIM_OS_UNIX_DEFS_H
 #define NVIM_OS_UNIX_DEFS_H
 
+// Windows doesn't have unistd.h, so we include it here to avoid numerous
+// instances of `#ifdef HAVE_UNISTD_H'.
 #include <unistd.h>
-#include <signal.h>
 
-// Defines BSD, if it's a BSD system.
-#ifdef HAVE_SYS_PARAM_H
-# include <sys/param.h>
-#endif
+// POSIX.1-2008 says that NAME_MAX should be in here
+#include <limits.h>
 
 #define TEMP_DIR_NAMES {"$TMPDIR", "/tmp", ".", "~"}
 #define TEMP_FILE_PATH_MAXLEN 256

src/nvim/os/win_defs.h

@@ -5,6 +5,8 @@
 #include <sys/stat.h>
 #include <stdio.h>
 
+#define NAME_MAX _MAX_PATH
+
 #define TEMP_DIR_NAMES {"$TMP", "$TEMP", "$USERPROFILE", ""}
 #define TEMP_FILE_PATH_MAXLEN _MAX_PATH