all: assorted cleanups and updates

Changes include:
  - add a bunch of "noexcept", mostly to operators and forceinline
  - use "uchar"
  - use "charptr"
  - rename options_t to Options
  - add ptr_check_no_overlap()
  - rewrite p_exe.cpp, NFCI
  - clang-format help.cpp
  - spelling fixes

src/options.h

@@ -29,6 +29,14 @@
 #ifndef UPX_OPTIONS_H__
 #define UPX_OPTIONS_H__ 1
 
+struct Options;
+extern Options *opt;
+#define options_t Options // old name
+
+#if WITH_THREADS
+extern std::mutex opt_lock_mutex;
+#endif
+
 /*************************************************************************
 // globals
 **************************************************************************/
@@ -46,7 +54,7 @@ enum {
     CMD_VERSION,
 };
 
-struct options_t final {
+struct Options final {
     int cmd;
 
     // compression options
@@ -164,8 +172,6 @@ struct options_t final {
     void reset();
 };
 
-extern struct options_t *opt;
-
 #endif /* already included */
 
 /* vim:set ts=4 sw=4 et: */