Disable symlinks: added explicit cast of AT_FDCWD (ticket #111).

Solaris has AT_FDCWD defined to unsigned value, and comparison of a file
descriptor with it causes warnings in modern versions of gcc.  Explicitly
cast AT_FDCWD to ngx_fd_t to resolve these warnings.

src/core/ngx_open_file_cache.c

diff --git a/src/core/ngx_open_file_cache.c b/src/core/ngx_open_file_cache.c
index acfc095..f42a16f 100644
--- a/src/core/ngx_open_file_cache.c
+++ b/src/core/ngx_open_file_cache.c
@@ -582,7 +582,7 @@
     p = name->data;
     end = p + name->len;
 
-    at_fd = AT_FDCWD;
+    at_fd = NGX_AT_FDCWD;
     at_name = *name;
 
     if (p[0] == '/') {
@@ -632,7 +632,7 @@
             goto failed;
         }
 
-        if (at_fd != AT_FDCWD && ngx_close_file(at_fd) == NGX_FILE_ERROR) {
+        if (at_fd != NGX_AT_FDCWD && ngx_close_file(at_fd) == NGX_FILE_ERROR) {
             ngx_log_error(NGX_LOG_ALERT, log, ngx_errno,
                           ngx_close_file_n " \"%V\" failed", &at_name);
         }
@@ -642,7 +642,7 @@
         at_name.len = cp - at_name.data;
     }
 
-    if (p == end && at_fd != AT_FDCWD) {
+    if (p == end && at_fd != NGX_AT_FDCWD) {
 
         /*
          * If pathname ends with a trailing slash, check if last path
@@ -690,7 +690,7 @@
 
 failed:
 
-    if (at_fd != AT_FDCWD && ngx_close_file(at_fd) == NGX_FILE_ERROR) {
+    if (at_fd != NGX_AT_FDCWD && ngx_close_file(at_fd) == NGX_FILE_ERROR) {
         ngx_log_error(NGX_LOG_ALERT, log, ngx_errno,
                       ngx_close_file_n " \"%V\" failed", &at_name);
     }

src/os/unix/ngx_files.h

diff --git a/src/os/unix/ngx_files.h b/src/os/unix/ngx_files.h
index 1526f9b..d397c2f 100644
--- a/src/os/unix/ngx_files.h
+++ b/src/os/unix/ngx_files.h
@@ -340,6 +340,8 @@
 
 #define ngx_file_at_info_n       "fstatat()"
 
+#define NGX_AT_FDCWD             (ngx_fd_t) AT_FDCWD
+
 #endif