Fix some warnings comparing char against EOF

This is not possible on platforms where char is unsigned.  In cases where fail() is checked, it is also unnecessary.

panda/src/downloader/documentSpec.cxx

@@ -66,7 +66,7 @@ input(std::istream &in) {
     // Scan the tag, up to but not including the closing paren.
     std::string tag;
     in >> ch;
-    while (!in.fail() && ch != EOF && ch != ')') {
+    while (!in.fail() && ch != ')') {
       tag += ch;
       // We want to include embedded whitespace, so we use get().
       ch = in.get();
@@ -81,7 +81,7 @@ input(std::istream &in) {
   // Scan the date, up to but not including the closing bracket.
   if (ch != ']') {
     std::string date;
-    while (!in.fail() && ch != EOF && ch != ']') {
+    while (!in.fail() && ch != ']') {
       date += ch;
       ch = in.get();
     }

panda/src/downloader/httpDate.cxx

@@ -279,7 +279,7 @@ input(std::istream &in) {
 
   string date;
   ch = in.get();
-  while (!in.fail() && ch != EOF && ch != '"') {
+  while (!in.fail() && ch != '"') {
     date += ch;
     ch = in.get();
   }

panda/src/pnmimagetypes/pnmFileTypePNM.cxx

@@ -193,7 +193,7 @@ pm_getuint(istream * const ifP) {
     'unsigned int'), issue an error message to stderr and abort the
     program.
     -----------------------------------------------------------------------------*/
-  char ch;
+  int ch;
   unsigned int i;
 
   // skip whitespace