feat: Implement AV evasion techniques (section renaming and padding)

This commit introduces two new AV evasion techniques:
- Section renaming: UPX0, UPX1, and UPX2 sections are now renamed to .text, .data, and .rdata respectively.
- Random padding: A new command-line option --add-padding is introduced to add random padding to the end of the packed file.

These changes aim to make UPX-packed executables harder to detect by antivirus software.

src/util/util.h

@@ -230,6 +230,7 @@ bool makebakname(char *ofilename, size_t size, const char *ifilename, bool force
 
 noinline bool is_envvar_true(const char *envvar, const char *alternate_name = nullptr) noexcept;
 
+void random_string(char *s, int len);
 unsigned get_ratio(upx_uint64_t u_len, upx_uint64_t c_len);
 bool set_method_name(char *buf, size_t size, int method, int level);
 void center_string(char *buf, size_t size, const char *s);